ImDrawList: Fix texture-based anti-aliasing with RGBA textures (#5132, #3245)

When using an Alpha8 font texture, GetTexDataAsRGBA32 converts 0x00 to transparent white.
When using a RGBA32 font texture, ImFontAtlasBuildRenderLinesTexData was writing transparent black.

docs/CHANGELOG.txt

@@ -61,6 +61,7 @@ Other Changes:
 - ColorEdit: Fixed text baseline alignment after a SameLine() after a ColorEdit() with visible label.
 - Stack Tool: Added option to copy item path to clipboard. (#4631)
 - Drawlist: Fixed PathArcTo() emitting terminating vertices too close to arc vertices. (#4993) [@thedmd]
+- DrawList: Fixed texture-based anti-aliasing path with RGBA textures (#5132, #3245) [@cfillion]
 - Misc: Fixed IsAnyItemHovered() returning false when using navigation.
 - Misc: Added IMGUI_STB_SPRINTF_FILENAME to support custom path to stb_sprintf. (#5068, #2954) [@jakubtomsu]
 - Misc: Added constexpr to ImVec2/ImVec4 inline constructors. (#4995) [@Myriachan]

imgui_draw.cpp

@@ -2738,13 +2738,13 @@ static void ImFontAtlasBuildRenderLinesTexData(ImFontAtlas* atlas)
         {
             unsigned int* write_ptr = &atlas->TexPixelsRGBA32[r->X + ((r->Y + y) * atlas->TexWidth)];
             for (unsigned int i = 0; i < pad_left; i++)
-                *(write_ptr + i) = IM_COL32_BLACK_TRANS;
+                *(write_ptr + i) = IM_COL32(255, 255, 255, 0);
 
             for (unsigned int i = 0; i < line_width; i++)
                 *(write_ptr + pad_left + i) = IM_COL32_WHITE;
 
             for (unsigned int i = 0; i < pad_right; i++)
-                *(write_ptr + pad_left + line_width + i) = IM_COL32_BLACK_TRANS;
+                *(write_ptr + pad_left + line_width + i) = IM_COL32(255, 255, 255, 0);
         }
 
         // Calculate UVs for this line