Codechange: Give sprite encoders a hint which colour components of a sprite are filled with useful information.

src/os/macosx/font_osx.cpp

@@ -286,6 +286,7 @@ const Sprite *CoreTextFontCache::InternalGetGlyph(GlyphID key, bool use_aa)
 	SpriteLoader::Sprite sprite;
 	sprite.AllocateData(ZOOM_LVL_NORMAL, width * height);
 	sprite.type = ST_FONT;
+	sprite.colours = (use_aa ? SCC_PAL | SCC_ALPHA : SCC_PAL);
 	sprite.width = width;
 	sprite.height = height;
 	sprite.x_offs = (int16)std::round(CGRectGetMinX(bounds));