gpu: fix negative intersections

Fixes a bug due to that f32.Rect.Intersect will not return the
empty rectangle for non intersecting rectangles - but instead
a swapped rectangle. By removing the .Canon() call in gpu.go we
ensure that non overlapping clipping rects and paint rects will
lead to no painting.

The Canon() call is not needed since boundsForTransformedRect()
was previously updated to always return a canonical rectangle.

Test case added.

Signed-off-by: Viktor <viktor.ogeman@gmail.com>

gpu/gpu.go

@@ -768,7 +768,7 @@ loop:
 			// the new bounding rectangle and the transformed original paint rectangle.
 			trans, off := splitTransform(state.t)
 			clipData, bnd, partialTrans := d.boundsForTransformedRect(op.Rect, trans)
-			clip := state.clip.Intersect(bnd.Add(off)).Canon()
+			clip := state.clip.Intersect(bnd.Add(off))
 			if clip.Empty() {
 				continue
 			}