testsuite: Add clip-translate-offscreen test

This was an experiment where an offscreen was translated inside an
existing clip.

Because renderers try to limit offscreens to the clip rect, this is
interesting, because they might get the translation wrong.

testsuite/gsk/compare/clip-translate-offscreen.node

@@ -0,0 +1,17 @@
+color {
+  bounds: 0 0 50 50;
+  color: rgb(0,0,0);
+}
+clip {
+  clip: 10 10 30 30;
+  child: transform {
+    transform: translate(10, 10);
+    child: opacity {
+      opacity: 0.8;
+      child: color {
+        bounds: -50 -50 100 100;
+        color: rgb(255,0,0);
+      }
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -31,6 +31,7 @@ compare_render_tests = [
   'clipped-repeat-3d-ngl',
   'clipped_rounded_clip',
   'clip-nested1',
+  'clip-translate-offscreen',
   'color-blur0',
   'color-matrix-identity',
   'color-matrix-parsing',