CollapsingHeader: Minor fix to align right side of frames (which is extruded past the Work/Contents rect) with clipping rectangle.

Extracted tab rendering code into a RenderTextEllipsis() function.

ImDrawListSplitter: Don't merge draw commands when crossing a VtxOffset boundary + Renamed fields ImDrawChannels to consistently suggest those are internal structures.

Window: child windows outer decorations (e.g. scrollbar) are rendered as part of their parent window, avoiding the creation of an extraneous draw command.
+ Metrics: inverted color of clip rect vs vertices bounding box when hovering a draw command, so the color matches the per-vertex preview.

Internals: Added drawlist and color arg to RenderArrow(), RenderBullet(). Reordered args for RenderPixelEllipsis.

Made PushID() behave the same in 32-bit and 64-bit, by not padding the integer into a void*. (Also technically faster.)

Synced/merged minor cruft from docking branch to minimize drift. AFAIK the only meaningful no-op change is that the call UpdateHoveredWindowAndCaptureFlags() was moved above UpdateMouseMovingNewFrame() to match what docking branch has been doing for a while.

ImDrawCallback: Allow to override the signature of ImDrawCallback by #define-ing it. This is meant to facilitate custom rendering back-ends passing local render-specific data to the draw callback.

Revert "Changed syntax for (very rarely used) IMGUI_OVERRIDE_DRAWVERT_STRUCT_LAYOUT mechanism, instead you only need to '#define ImDrawVert MyDrawVert' to use this feature, avoiding the need to declare the entire structure within an awkward macro. Using the old macro will now error with a message pointing you to the new method. (#38, #103, #1172, #1231, #2489)"

This reverts commit 597c0249.

Changed syntax for (very rarely used) IMGUI_OVERRIDE_DRAWVERT_STRUCT_LAYOUT mechanism, instead you only need to '#define ImDrawVert MyDrawVert' to use this feature, avoiding the need to declare the entire structure within an awkward macro. Using the old macro will now error with a message pointing you to the new method. (#38, #103, #1172, #1231, #2489)

Window: Fixed one case where auto-resize by double-clicking the resize grip would make either scrollbar appear for a single frame after the resize. Moved Scrollbar visibility block.

Internals: Moved scrollbar visibility calculation block below the call to UpdateManualResize(). This commit is _intended_ to have no side-effect (next commit will). Also moved ItemWidthDefault calculation below rectangles.

Scrollbar overlap an extra WindowBorderSize amount on the left to make all distances consistent. Reverted to BorderSize not affecting work/contents rectangles. Scrollbar, Style: Changed default style.ScrollbarSize from 16 to 14.

Internals: Minor no-op tidying up toward solving the WindowPadding / WindowBorderSize / ScrollbarSize overlapping mess.
+ Demo: Use SetScrollY().

Fixed newly created window (e.g. appearing child window) from having scrollbar active on the first frame. (fix 6e03b27) + reworded code a little. (+1 squashed commits)
Fixed auto-resize with AlwaysVerticalScrollbar or AlwaysHorizontalScrollbar flags not taking account of the expect scrollbar sizes.

Internals: Storing ScrollMax into a member. Mostly to facilitate debugging. Also locking down window->Scroll slightly lower in the Begin function.

TextWrapped, PushTextWrapPos(0.0f) within a window with horizontal scrolling from not covering the full horizontal area (previously only worked with an explicit contents size).

Internals: Renamed SizeContents to ContentSize, SizeContentsExplicit to ContentSizeExplicit. Tweaked Metrics->Show Rectangles functionality.

Window rectangles: Changed WorkRect to cover the whole region including scrolling (toward obsolete ContentsRegionRect) + using full WindowPadding*1 padding.
Tweaked InnerClipRect.
TreeNode, CollapsingHeader: Fixed highlight frame not covering horizontal area fully when using horizontal scrolling. (#2211, #2579)
TabBar: Fixed BeginTabBar() within a window with horizontal scrolling from creating a feedback loop with the horizontal contents size.
Columns: Fixed Columns() within a window with horizontal scrolling from not covering the full horizontal area (previously only worked with an explicit contents size). (#125)
Demo: Added demo code to test contentsrect/workrect

Clarified behavior of SetNextWindowContentSize(). Content size is defined as the size available after removal of WindowPadding on each sides. So SetNextWindowContentSize(ImVec2(100,100)) + auto-resize will always allow submitting a 100x100 item without creating a scrollbar, regarding of WindowPadding.The exact meaning of ContentSize for decorated windows was previously ill-defined.

Window rectangles: Made InnerRect not affected by window border sizes. its few users shouldn't be meaningfully affected.

Comments about obsoleted features version. Todo. Clarify tab bar initial offset (useful if we decide to remove the half-windowpadding clip margin).

Nav: Fixed rare crash when e.g. releasing Alt-key while focusing a window with a menu at the same frame as clearing the focus. This was in most noticeable in some back-ends with emits key release events when focusing another viewport. (#2609)

Nav: Fixed gamepad/keyboard moving of window affecting contents size incorrectly, sometimes leading to scrollbars appearing during the movement. + minor fix with the mostly dead Ctrl+wheel scaling.

ImDrawListSplitter: Support merging consecutive draw commands straddling two channels. Support zero-init.

ImDrawListSplitter: extracted out of ImDrawList. Down the line we may obsolete the ImDrawList functions and encourage users to store the splitter aside, in the meanwhile ImDrawList holds a splitter.
(This will allow columns/table to recurse.)

ImDrawList: Fix broken channel splitting (another issue when the first channel is empty) (#2591) + fixed warnings with newer VS

Fixes compilation with devkitPro for Nintendo Switch

Internals: Window rectangles: Fixed ContentsRegion lag by moving back after Scrollbar, fixes b50c61c9. Shuffling setup order and added comments.

Internals: Window rectangles: Renaming of all rectangles toward their final form. Should be a no-op. Renamed GetWorkRectMax() to GetContentRegionMaxAbs(). Metrics shows SizeContents.

Examples/Backends: Metal: Added support for large meshes (64k+ vertices) with 16-bits indices, enable 'ImGuiBackendFlags_HasVtxOffset' config flag in back-end. (#2591, #2592)

Renamed ImGuiBackendFlags_HasVtxOffset to ImGuiBackendFlags_RendererHasVtxOffset to match naming convention already used in viewport/docking branch. (#2591) + Fix OpenGL3 code missing flag.