testsuite: Add a nonorthogonal linear-gradient transform

The actual gradient line is covered by blocks, so there are no
artifacts. But if a renderer screws this up, the blue/red will seep
through these blocks.

testsuite/gsk/compare/linear-gradient-nonorthogonal-scale.node

@@ -0,0 +1,29 @@
+transform {
+  transform: scale(2, 1);
+  child: linear-gradient {
+    bounds: 0 0 50 50;
+    start: 0 0;
+    end: 50 50;
+    stops: 0.5 rgb(255,0,0), 0.5 rgb(0,0,255);
+  }
+}
+color {
+  bounds: 0 39 22 11;
+  color: rgb(0,0,0);
+}
+color {
+  bounds: 18 29 24 12;
+  color: rgb(0,0,0);
+}
+color {
+  bounds: 38 19 24 12;
+  color: rgb(0,0,0);
+}
+color {
+  bounds: 58 9 24 12;
+  color: rgb(0,0,0);
+}
+color {
+  bounds: 78 0 22 11;
+  color: rgb(0,0,0);
+}

testsuite/gsk/meson.build

@@ -71,6 +71,7 @@ compare_render_tests = [
   'inset-shadow-multiple',
   'invalid-transform',
   'issue-3615',
+  'linear-gradient-nonorthogonal-scale',
   'linear-gradient-with-64-colorstops',
   'mask',
   'mask-clipped-inverted-alpha',