testsuite: Add a test for a clipping cornercase

If the new clip cannot intersect with the old clip and there is a global
opacity setting, things can go wrong.

So test that they don't.

testsuite/gsk/compare/clip-intersection-fail-opacity-nogl.node

@@ -0,0 +1,30 @@
+color {
+  bounds: 0 0 50 50;
+  color: rgb(0,255,0);
+}
+opacity {
+  opacity: 0.2;
+  child: rounded-clip {
+    clip: 0 0 50 50 / 50 0 50 0;
+    child: clip {
+      clip: 2.5 2.5 45 45;
+      child: shadow {
+        shadows: rgb(0,0,0) 20 20, rgb(0,0,0) -20 -20;
+        child: container {
+          color {
+            bounds: 30 10 10 10;
+            color: rgb(255,0,0);
+          }
+          color {
+            bounds: 20 20 10 10;
+            color: rgb(255,0,0);
+          }
+          color {
+            bounds: 10 30 10 10;
+            color: rgb(255,0,0);
+          }
+        }
+      }
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -32,6 +32,7 @@ compare_render_tests = [
   'clip-in-rounded-clip1',
   'clip-in-rounded-clip2',
   'clip-in-rounded-clip3',
+  'clip-intersection-fail-opacity-nogl',
   'clipped-repeat',
   'clipped_rounded_clip',
   'clip-nested1',