CollapsingHeader: Added support for ImGuiTreeNodeFlags_Bullet and ImGuiTreeNodeFlags_Leaf on framed nodes. (#2159, #2160)

The Bullet and Leaf ImGuiTreeNodeFlags are now taken into account for Framed/CollapsingHeader tree nodes as well. TreeNodeEx() can be used to specify these flags. A choice was made to left-adjust the Framed text when no Bullet/Arrow is rendered, since this was deemed to look better in the Framed context (especially when considering that CollapsingHeader is drawn using NoTreePushOnOpen, so child/sibling Text items etc will often be non-indented).

Backends: OpenGL3: Attempt to automatically detect default GL loader by using __has_include. Followup to 44cd8e39 (#2798)

TabBar: Fixed single tab shrinking reducing the tab to 0.0f size. Broken by a856c670.

Selectable: Added ImGuiSelectableFlags_AllowItemOverlap flag in public api (was previously internal only).

ColorPicker / ColorEdit: restore Hue when zeroing Saturation. (#2722, #2770) - changelog, fixed uninitialized variables, tweaks, renaming.

Issue is fixed by storing last active color picker color and last hue value when active color picker takes rgb as input. Then if current color picker color matches last active color - hue value will be restored. IDs are not used because ColorEdit4() and ColorWidget4() may call each other in hard-to-predict ways and they both push their own IDs on to the stack. We need hue restoration to happen in entire stack of these widgets if topmost widget used hue restoration. Since these widgets operate on exact same color value - color was chosen as a factor deciding which widgets should restore hue.

TreeNode: Added ImGuiTreeNodeFlags_SpanAvailWidth and ImGuiTreeNodeFlags_SpanFullWidth flags (#2451, #2438, #1897)
Added demo bits.

ImDrawListSplitter: fixed an issue merging channels if the last submitted draw command used a different texture. (#2506)

Style: Allow style.WindowMenuButtonPosition to be set to ImGuiDir_None to hide the collapse button. (#2634, #2639)
+ Fix #2775

# Conflicts:
#	imgui.cpp

Font: Narrow ellipsis: once we know an ellipsis is going to be drawn, we can claim the space between pos_max.x and ellipsis_max.x which gives us enough extra space to not requires the further (and otherwise valid) optimizations. Gets us vastly simplified code, yay. (#2775)

This changeset implements several pieces of the puzzle that add up to a narrow ellipsis rendering.

## EllipsisCodePoint

`ImFontConfig` and `ImFont` received `ImWchar EllipsisCodePoint = -1;` field. User may configure `ImFontConfig::EllipsisCodePoint` a unicode codepoint that will be used for rendering narrow ellipsis. Not setting this field will automatically detect a suitable character or fall back to rendering 3 dots with minimal spacing between them. Autodetection prefers codepoint 0x2026 (narrow ellipsis) and falls back to 0x0085 (NEXT LINE) when missing. Wikipedia indicates that codepoint 0x0085 was used as ellipsis in some older windows fonts. So does default Dear ImGui font. When user is merging fonts - first configured and present ellipsis codepoint will be used, ellipsis characters from subsequently merged fonts will be ignored.

## Narrow ellipsis

Rendering a narrow ellipsis is surprisingly not straightforward task. There are cases when ellipsis is bigger than the last visible character therefore `RenderTextEllipsis()` has to hide last two characters. In a subset of those cases ellipsis is as big as last visible character + space before it. `RenderTextEllipsis()` tries to work around this case by taking free space between glyph edges into account. Code responsible for this functionality is within `if (text_end_ellipsis != text_end_full) { ... }`.

## Fallback (manually rendered dots)

There are cases when font does not have ellipsis character defined. In this case RenderTextEllipsis() falls back to rendering ellipsis as 3 dots, but with reduced spacing between them. 1 pixel space is used in all cases. This results in a somewhat wider ellipsis, but avoids issues where spaces between dots are uneven (visible in larger/monospace fonts) or squish dots way too much (visible in default font where dot is essentially a pixel). This fallback method obsoleted `RenderPixelEllipsis()` and this function was removed. Note that fallback ellipsis will always be somewhat wider than it could be, however it will fit in visually into every font used unlike what `RenderPixelEllipsis()` produced.

BeginTabItem: Fixed case where right-most tab would create an extraneous draw calls (probably related to other tab fitting code in 1.73 wip)

- append CXXFLAGS instead of overwriting them
- add glad.c build rule

Columns: Separator: Fixed a bug where non-visible separators within columns would alter the next row position differently than visible ones.
Fixed rounding issues also leading to change of ScrollMax depending on visible items (in particular negative coordinate would be rounded differently)

decreasing the value was broken on arm64

Backends: OpenGL3: Tweaked initialization code allow application calling ImGui_ImplOpenGL3_CreateFontsTexture() before ImGui_ImplOpenGL3_NewFrame() if for some reason they wanted.

TabBar: feed desired width (sum of unclipped tabs width) into layout system to allow for auto-resize. (#2768)
Before 1.71 tab bars fed the sum of current width which created feedback loops in certain situations. Amend f95c77ee.

TabBar: improved shrinking for large number of tabs to avoid leaving extraneous space on the right side. Individuals tabs are given integer-rounded width and remainder is spread between tabs left-to-right.