Font: Narrow ellipsis: various minor stylistic tweaks (#2775)

Font: implement a way to draw narrow ellipsis without relying on hardcoded 1 pixel dots. (#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)

Examples: SDL/GLFW + OpenGL3: Fixes for Makefile (#2774) ...

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

Fix signed types warning in pasteboard handler (#2786)

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)

Nav, Scrolling: Added support for Home/End key. (#787)

Fix DragScalar for unsigned types (#2780) ...

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.

Fixed missing IMGUI_API for IsMouseDragPastThreshold().

ColorEdit: Disable Hue edit when Saturation==0 instead of letting Hue values jump around.

DragInt, DragFloat, DragScalar: Using (v_min > v_max) allows locking any edit to the value.

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 f95c77e.

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.

Disable with ConfigWindowsMemoryCompactTimer < 0.0f (#2636)

Added a mechanism to compact/free the larger allocations of unused windows (buffers are compacted when a window is unused for 60 seconds, as per io.ConfigWindowsMemoryCompactTimer = 60.0f). Note that memory usage has never been reported as a problem, so this is merely a touch of overzealous luxury. (#2636)

Demo: PlotLine example displays the average value. (#2759) + extra comments

SliderScalar: Improved assert when using U32 or U64 types with a large v_max value. (#2765) ...

+  misc minor stuff.

Rebased imstb_rectpack on stb_rect_pack v1.00.

Internals: Storing settings using ImVec2ih to match what we are doing with dock node. + removed ImMax from reading Size value (done in Begin) + removed seemingly unnecessary FLT_MAX compare in SettingsHandlerWindow_WriteAll. ...

About: Added backquote to text copied into clipboard so it doesn't mess up with github formatting when pasted.

ImDrawList: clarified the name of many parameters so reading the code is a little easier. (#2740)

Using offsetof() when available in C++11. Avoids Clang sanitizer complaining about old-style macros. (#94)

More typos in comments (#2738)

TabBar: fixed single-tab not shrinking their width down. ...

+ minor typo fixes (#2738)

Internals: Renaming window size calc functions.

TabBar: fixed ScrollToBar request creating bouncing loop when tab is larger than available space.

ColorPicker: Made rendering aware of global style alpha of the picker can be faded out. (#2711) ...

Note that some elements won't accurately fade down with the same intensity, and the color wheel when enabled will have small overlap glitches with (style.Alpha < 1.0).

Removing Funding file (unnecessary as we'll switch services)

Create FUNDING.yml

Explicit narrowing cast from size_t to UINT (#2726) ...

Clang: `non-constant-expression cannot be narrowed from type 'size_t' (aka 'unsigned long long') to 'UINT' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]`