Add a test for mask nodes

testsuite/gsk/compare/mask.node

@@ -0,0 +1,20 @@
+mask {
+  source: container {
+    color {
+      color: red;
+      bounds: 0 0 50 50;
+    }
+    color {
+      color: yellow;
+      bounds: 50 0 50 50;
+    }
+    color {
+      color: magenta;
+      bounds: 0 50 100 50;
+    }
+  }
+  mask: color {
+    color: black;
+    bounds: 20 20 50 50;
+  }
+}

testsuite/gsk/meson.build

@@ -54,6 +54,7 @@ compare_render_tests = [
   'inset-shadow-multiple',
   'invalid-transform',
   'issue-3615',
+  'mask',
   'nested-rounded-clips',
   'opacity_clip',
   'opacity-overdraw',