Address review comments.

wrench/reftests/mask/checkerboard-tiling.yaml

@@ -1,3 +1,4 @@
+# Tests a tiled image mask with leftover tile offsets.
 ---
 root:
   items:
@@ -7,7 +8,8 @@ root:
         image: transparent-checkerboard(2, 16, 16)
         rect: [0, 0, 200, 200]
         repeat: false
-        tile-size: 37 # Intentional
+        # This is the point of the test, can't be a divisor of 200.
+        tile-size: 37
       items:
       - type: rect
         bounds: [0, 0, 200, 200]