Fill in slightly more code for the image testing.

This still needs a real solution, but just want to get it past
clippy.

vello_tests/tests/property.rs

@@ -55,16 +55,19 @@ fn empty_scene(use_cpu: bool) {
     };
     let image = vello_tests::render_then_debug_sync(&scene, &params).unwrap();
     assert_eq!(image.format, Format::Rgba8);
-    // FIXME(color): Enable when there is a solution for PartialEq
-    /*
     for pixel in image.data.data().chunks_exact(4) {
         let &[r, g, b, a] = pixel else { unreachable!() };
         let image_color = Color::from_rgba8(r, g, b, a);
-        if image_color != color {
-            panic!("Got {image_color:?}, expected clear colour {color:?}");
+        // FIXME(color): Enable when there is a solution for PartialEq. This should
+        // not be discarding alpha, but there's no AlphaColor::difference.
+        if image_color
+            .discard_alpha()
+            .difference(color.discard_alpha())
+            < 1e-4
+        {
+            panic!("Got {image_color:?}, expected clear color {color:?}");
         }
     }
-    */
 }
 
 #[test]