testsuite: Add a test for mask out of bounds effects

Inverted alpha masks have an effect on the source, even if the mask
doesn't cover the source at all - or worse, is completely clipped out.

The GL renderer handles this fine, but Cairo and Vulkan had
optimizations that got this wrong.

testsuite/gsk/compare/mask-clipped-inverted-alpha.node

@@ -0,0 +1,14 @@
+clip {
+  clip: 0 0 40 40;
+  child: mask {
+    mode: inverted-alpha;
+    source: color {
+      bounds: 0 0 100 100;
+      color: rebeccapurple;
+    }
+    mask: color {
+      bounds: 40 40 60 60;
+      color: black;
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -65,6 +65,7 @@ compare_render_tests = [
   'invalid-transform',
   'issue-3615',
   'mask',
+  'mask-clipped-inverted-alpha',
   'mask-modes',
   'mask-modes-with-alpha',
   'nested-rounded-clips',