renderer: fix color glyph rendering under OpenGL

Also changes color atlas to always use an sRGB internal format so that
the texture reads automatically linearize the colors.

Renames the misleading `rgba` atlas format to `bgra`, since both
FreeType and CoreText are set up to draw color glyphs in bgra.

src/renderer/generic.zig

@@ -336,7 +336,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                 const color = try api.initAtlasTexture(&.{
                     .data = undefined,
                     .size = 1,
-                    .format = .rgba,
+                    .format = .bgra,
                 });
                 errdefer color.deinit();