Menus: Fixed an assertion happening in some situations when closing nested menus (broken in 1.83). (#4640)

Broken since 936f5322
Weirdly chain-reaction caused by the fact following #4640 repro, the SourceWindow assignment in OpenPopupEx() picks Menu_04 before its closure. Value of SourceWindow since bda2cde6 #2517

SetItemDefaultFocus() use ScrollToRectEx(), don't tab when Alt is held either, TabItemEx() uses ItemAdd's extra_flags, misc comments.

Backends: SDL_Renderer: use SDL_RenderIsClipEnabled() to pass NULL to SDL_RenderSetClipRect (#3926, #4582, #4633)

Nav: refactor SetKeyboardFocusHere() into using Nav facility. Fix it for clipped items. (#343, #4079, #2352, #432)

+ Removed references to counter used by previous implementation of SetKeyboardFocus functions (the TabStop ones will be removed after)

Stack Tool: Added Stack Tool (ShowStackToolWindow() function and available from Demo and Metrics window). (#4631)

Moved !IsActiveIdUsingKey(ImGuiKey_Tab) check to UpdateTabFocus(). Added asserts to IsItemHovered() and IsWindowHovered().

Internal: added ScrollToItem() upcoming replacement to ScrollToBringRectIntoView() + ImGuiScrollFlags (WIP) (#3692, #3208, #2812, #4242, #2900)

Added ClosePopupsExceptModals() helper, unused for now (aimed at user being able to close popups on app focus loss, not necessarily a suitable default)

IO: modify io.AddFocusEvent() to tolerate in/out for multi-viewports. Amend 2f40be63. (merged from docking) (#3532)

(Squashed 20 commits)

ColorPicker: Fix not being able to pick exactly (1.0f, 1.0f, 1.0f) color by dragging toward the edges of the SV square. (#3517)

Old code attempted to mitigate hue/saturation resetting for colors where these components are undefined. Since we now explicitly back up and restore these components this workaround is no longer necessary.

* Change g.ColorEditLastColor type to ImU32 and store RGB color value.
  - Fixes inability to change hue when saturation is 0. (#4014)
  - Fixes edgecases where lossy color conversion prevent restoration of hue/saturation.
  - Fixes hue value jitter when modifying color using SV square.
* Fix hue resetting to 0 when it is set to 255 by explicitly restoring hue if it is 0 and previous value was 1.
* Further reduce hue jitter by restoring hue when color is modified using SV square.

Nav: Fixed an issue with losing focus on docked windows when pressing Alt while keyboard navigation is disabled. (#4547, #4439)

Backends: OSX: Use mach_absolute_time as CFAbsoluteTimeGetCurrent can jump backwards. (#4557, #4563)

Backends: Added more implicit asserts to detect invalid/redundant calls to Shutdown functions. (#4562)

InputText: fix Space key with nav enabled interfering with input text boxes (fix bd6c9e99). made it possible to activate InputText with tweak gamepad button (why not, now that we can cancel) (#4552, #2321)

Disabled: Added assert guard for mismatching BeginDisabled()/EndDisabled() blocks. (#211) + Added asserts for missing PopItemFlag() calls. Added both to ErrorCheckEndFrameRecover (#1651)

Menus: Fixed vertical alignments of MenuItem() calls within a menu bar. (broken by f8fae022). (#4538)

Nav: Fixed a few widgets from not setting reference keyboard/gamepad navigation ID when activated with mouse.

Internals: refactored IsWindowHovered()/IsWindowFocused() to make their logic more similar + change underlying value of ImGuiHoveredFlags_AllowWhenBlockedByPopup + comment out docking only flags.

(this commit should have no visible side effect but is designed to introduce the followup commit refactoring SetKeyboardFocusHere into using a Nav request)

Backends: OpenGL3: Made our custom GL loader a bit more tolerant of use of another loader in same compilation unit. (#4445, #4514)