testsuite: Add a test for proper depth handling

Create a 16bit texture with a color values of
  color(srgb 0.999 0 0)
And then check that diffing it with a color node with that same color
returns black.
A color-matrix node has been added to multiply the difference so that it
can't escape due to being < 1/255.

testsuite/gsk/compare/bug-rgba8-offscreen-in-high-depth.node

@@ -0,0 +1,18 @@
+color-matrix {
+  matrix: matrix3d(65535, 0, 0, 0, 0, 65535, 0, 0, 0, 0, 65535, 0, 0, 0, 0, 1);
+  child: blend {
+    mode: difference;
+    bottom: texture {
+      bounds: 0 0 10 10;
+      texture: url("data:image/png;base64,\
+iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKEAYAAADdohP+AAAAAXNSR0IArs4c6QAAAARjSUNQAQ0A\
+AZxpOzIAAAAfSURBVCiRY/w/n4GBgeH/fwYqASZqGTRq4KiBg8lAAH3dA7DFgV4pAAAAAElFTkSu\
+QmCC\
+");
+    }
+    top: color {
+      bounds: 0 0 10 10;
+      color: color(srgb 0.999 0 0);
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -31,6 +31,7 @@ compare_render_tests = [
   'border-zero-width-color',
   'borders-rotated',
   'borders-scaled',
+  'bug-rgba8-offscreen-in-high-depth',
   'clip-contained',
   'clip-coordinates-2d',
   'clip-coordinates-nocairo',