testuite: Add opacity-overdraw test

Tests that overdrawing of content inside an opacity node happens before
the opacity is applied.

This is broken in the GL renderer and causes the opacity.ui reftest to
fail.

testsuite/gsk/compare/opacity-overdraw.node

@@ -0,0 +1,15 @@
+opacity {
+  child: transform {
+    transform: translate(15, 15);
+    child: container {
+      color {
+        bounds: -15 -15 30 30;
+        color: rgb(0,0,255);
+      }
+      color {
+        bounds: -10 -10 20 20;
+        color: rgb(255,0,0);
+      }
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -62,6 +62,7 @@ compare_render_tests = [
   'issue-3615',
   'nested-rounded-clips',
   'opacity_clip',
+  'opacity-overdraw',
   'opacity-overlapping-children',
   'outset_shadow_offset_both',
   'outset_shadow_offset_x',