testsuite: Add a test for repeating gradients

Make sure scaling the image also scales the color stop lookup.

testsuite/gsk/compare/repeating-gradient-scaled.node

@@ -0,0 +1,9 @@
+transform {
+  transform: scale(10);
+  child: repeating-linear-gradient {
+    bounds: 0 0 5 5;
+    start: 0 0;
+    end: 0 1;
+    stops: 0 white, 0.5 white, 0.5 black, 1 black;
+  }
+}

testsuite/gsk/meson.build

@@ -80,6 +80,7 @@ compare_render_tests = [
   'repeat-empty-child-bounds',
   'repeat-negative-coords',
   'repeat-texture',
+  'repeating-gradient-scaled',
   'scale-textures-negative-ngl',
   'scale-up-down',
   'scaled-cairo',