testsuite: Add tests for empty mask children

The compare tests use an empty container node, but running them with
--replay ends up with empty nodes in snapshots due to how containers are
replayed.

Related: !7396
Related: #6761

testsuite/gsk/compare/mask-empty-mask.node

@@ -0,0 +1,35 @@
+mask {
+  source: color {
+    bounds: 0 0 40 40;
+    color: rgb(255,0,204);
+  }
+  mask: container {
+  }
+}
+mask {
+  mode: inverted-alpha;
+  source: color {
+    bounds: 0 50 40 40;
+    color: rgb(255,0,204);
+  }
+  mask: container {
+  }
+}
+mask {
+  mode: luminance;
+  source: color {
+    bounds: 50 0 40 40;
+    color: rgb(255,0,204);
+  }
+  mask: container {
+  }
+}
+mask {
+  mode: inverted-luminance;
+  source: color {
+    bounds: 50 50 40 40;
+    color: rgb(255,0,204);
+  }
+  mask: container {
+  }
+}

testsuite/gsk/compare/mask-empty-source.node

@@ -0,0 +1,41 @@
+/* guarantee bounds */
+color {
+  color: transparent;
+  bounds: 0 0 90 90;
+}
+
+mask {
+  source: container {
+  }
+  mask: color {
+    bounds: 0 0 40 40;
+    color: rgb(255,0,204);
+  }
+}
+mask {
+  mode: inverted-alpha;
+  source: container {
+  }
+  mask: color {
+    bounds: 0 50 40 40;
+    color: rgb(255,0,204);
+  }
+}
+mask {
+  mode: luminance;
+  source: container {
+  }
+  mask: color {
+    bounds: 50 0 40 40;
+    color: rgb(255,0,204);
+  }
+}
+mask {
+  mode: inverted-luminance;
+  source: container {
+  }
+  mask: color {
+    bounds: 50 50 40 40;
+    color: rgb(255,0,204);
+  }
+}

testsuite/gsk/meson.build

@@ -99,6 +99,8 @@ compare_render_tests = [
   'linear-gradient-with-64-colorstops',
   'mask',
   'mask-clipped-inverted-alpha',
+  'mask-empty-mask',
+  'mask-empty-source',
   'mask-modes',
   'mask-modes-in-opacity',
   'mask-modes-with-alpha',