testsuite: Add a test for repeat node offscreen scaling

Ensure that the offscreens for repeat nodes pick the right xscale and
yscale so that they render pixel-aligned.

testsuite/gsk/compare/repeat-scaling.node

@@ -0,0 +1,13 @@
+transform {
+  transform: matrix(0, 4,
+                   -8, 0,
+                    0, 0);
+  child: repeat {
+    bounds: 0 0 5 2.5;
+    child-bounds: 0 0 1 0.5;
+    child: color {
+      bounds: 0.25 0.125 0.5 0.25;
+      color: rgb(0,0,255);
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -96,6 +96,7 @@ compare_render_tests = [
   'repeat-no-repeat',
   'repeat-repeats',
   'repeat-repeats-nested',
+  'repeat-scaling',
   'repeat-texture',
   'repeating-gradient-scaled',
   'scale-textures-negative-ngl',