gsk: Add a testcase for underlines and carets

The gl renderer has an optimization where it uses the glyph atlas
to render color nodes that show up in the middle of text (e.g. for
underlines and carets). This adds a simple test for that scenario,
which hits this codepath.

testsuite/gsk/compare/text-color-mix.node

@@ -0,0 +1,21 @@
+text {
+  font: "text-mixed-color-colrv1 15" url("data:font/ttf;base64,AAEAAAAKAIAAAwAgQ09MUhc9T40AAAI4AAAAe0NQQUwB/wATAAACtAAAABpjbWFwAHUAPQAAATgA\
+AAA0Z2x5Zn7NhtgAAAF8AAAAkGhlYWQmof0NAAAArAAAADZoaGVhDAEEAgAAAOQAAAAkaG10eAQA\
+AAAAAAEoAAAAEGxvY2EAbACQAAABbAAAABBtYXhwAAkABQAAAQgAAAAgbmFtZR9CFpQAAAIMAAAA\
+KQABAAAAARmaAQnTZV8PPPUAAggAAAAAAOHCPQAAAAAA4cpZfQAAAAAEAAgAAAAAAQACAAAAAAAA\
+AAEAAAgAAAAAAAQAAAAAAAQAAAEAAAAAAAAAAAAAAAAAAAABAAEAAAAHAAQAAQAAAAAAAQAAAAAA\
+AAAAAAAAAAAAAAAEAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAMAAAAMAAQAKAAAAAYABAABAAIAIABG\
+//8AAAAgAEH////g/8AAAQAAAAAAAAAAAAAADAAYACQAMAA8AEgAAQAAAAAEAAgAAAMAADEhESEE\
+APwACAAAAQAAAAAEAAgAAAMAADEhESEEAPwACAAAAQAAAAAEAAgAAAMAADEhESEEAPwACAAAAQAA\
+AAAEAAgAAAMAADEhESEEAPwACAAAAQAAAAAEAAgAAAMAADEhESEEAPwACAAAAQAAAAAEAAgAAAMA\
+ADEhESEEAPwACAAAAAABABIAAQAAAAAAAQAXAAB0ZXh0LW1peGVkLWNvbG9yLWNvbHJ2MQAAAAAB\
+AAAAAAAAAAAAAAAAAAAAIgAAAEoAAAAAAAAAAAAAAAAAAAADAAQAAAAWAAUAAAAcAAYAAAAiAQEA\
+AAAAAQEAAAABAQEAAAACAAAAAwAAABAAAAAbAAAAJgoAAAYAAQIAAEAACgAABgACAgABQAAKAAAG\
+AAMCAAJAAAAAAAADAAEAAwAAAA4AAAAA//8A/wD//wAA/wAA\
+");
+  glyphs: 1 20;
+}
+color {
+  bounds: 0 5 20 5;
+  color: rgb(255,0,0);
+}

testsuite/gsk/meson.build

@@ -136,6 +136,7 @@ compare_render_tests = [
   'stroke-fractional-translate-nogl',
   'stroke-opacity',
   'stroke-with-3d-contents-nogl-nocairo',
+  'text-color-mix',
   'text-glyph-lsb',
   'text-mixed-color-nocairo',
   'text-mixed-color-colrv1',