imgui_freetype: fixed a packing issue which in some occurrences would prevent large amount of glyphs from being packed correctly. (#5788, #5829)

This seemingly innocuous change sursingly had very large side-effects of completly breaking packing for the test font mentioned in above issue. Not even sure why tbh. New code matches what stb_truetype's stbtt_PackBegin() does.

docs/CHANGELOG.txt

@@ -67,6 +67,8 @@ Other changes:
 - Misc: added GetItemID() in public API. It is not often expected that you would use this,
   but it is useful for Shortcut() and upcoming owner-aware input functions which wants to
   be implemented with public API.
+- Fonts: imgui_freetype: fixed a packing issue which in some occurrences would prevent large
+  amount of glyphs from being packed correctly. (#5788, #5829)
 - Fonts: added a 'void* UserData' field in ImFontAtlas, as a convenience for use by
   applications using multiple font atlases.
 - Demo: added "Inputs->Shortcut Routing" section.

misc/freetype/imgui_freetype.cpp

@@ -6,6 +6,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2023/01/04: fixed a packing issue which in some occurrences would prevent large amount of glyphs from being packed correctly.
 //  2021/08/23: fixed crash when FT_Render_Glyph() fails to render a glyph and returns NULL.
 //  2021/03/05: added ImGuiFreeTypeBuilderFlags_Bitmap to load bitmap glyphs.
 //  2021/03/02: set 'atlas->TexPixelsUseColors = true' to help some backends with deciding of a prefered texture format.
@@ -586,7 +587,7 @@ bool ImFontAtlasBuildWithFreeTypeEx(FT_Library ft_library, ImFontAtlas* atlas, u
     ImVector<stbrp_node> pack_nodes;
     pack_nodes.resize(num_nodes_for_packing_algorithm);
     stbrp_context pack_context;
-    stbrp_init_target(&pack_context, atlas->TexWidth, TEX_HEIGHT_MAX, pack_nodes.Data, pack_nodes.Size);
+    stbrp_init_target(&pack_context, atlas->TexWidth - atlas->TexGlyphPadding, TEX_HEIGHT_MAX - atlas->TexGlyphPadding, pack_nodes.Data, pack_nodes.Size);
     ImFontAtlasBuildPackCustomRects(atlas, &pack_context);
 
     // 6. Pack each source font. No rendering yet, we are working with rectangles in an infinitely tall texture at this point.