Unicode: Changelog, comments, minimum CI integration. (#2541, #2538, #2815)

docs/CHANGELOG.txt

@@ -45,6 +45,15 @@ Other Changes:
   when the menu is not open. (#3030)
 - InputText: Fixed password fields displaying ASCII spaces as blanks instead of using the '*'
   glyph. (#2149, #515)
+- Added optional support for Unicode plane 1-16 (#2538, #2541, #2815) [@cloudwu, @samhocevar]
+  - Compile-time enable with '#define ImWchar ImWchar32' in imconfig.h.
+  - Generally more consistent support for unsupported codepoints (0xFFFD), in particular when
+    using the default, non-fitting characters will be turned into 0xFFFD instead of being ignored.
+  - Surrogate pairs are supported when submitting UTF-16 data via io.AddInputCharacterUTF16(),
+    allowing for more complete CJK input.
+  - sizeof(ImWchar) goes from 2 to 4. IM_UNICODE_CODEPOINT_MAX goes from 0xFFFF to 0x10FFFF.
+  - Various structures such as ImFont, ImFontGlyphRangesBuilder will use more memory, this
+    is currently not particularly efficient.
 - Window: Fixed a bug with child window inheriting ItemFlags from their parent when the child
   window also manipulate the ItemFlags stack. (#3024) [@Stanbroek]
 - Font: Fixed non-ASCII space occasionally creating unnecessary empty polygons.
@@ -53,6 +62,8 @@ Other Changes:
   ImGui_ImplWin32_GetDpiScaleForMonitor() helpers functions (backported from the docking branch).
   Those functions makes it easier for example apps to support hi-dpi features without setting up
   a manifest.
+- Backends: Win32: Calling AddInputCharacterUTF16() from WM_CHAR message handler in order to support
+  high-plane surrogate pairs. (#2815) [@cloudwu, @samhocevar]
 - Backends: SDL: Added ImGui_ImplSDL2_InitForMetal() for API consistency (even though the function
   currently does nothing).
 - Backends: SDL: Fixed mapping for ImGuiKey_KeyPadEnter. (#3031) [@Davido71]