Add unit test of op chaining.
Relax bounds checks in op merging/chaining to only check bounds against
heads of op chains.
Change-Id: I714435913b901c0a068bc7233ca30f2ab7916c2e
Reviewed-on: https://skia-review.googlesource.com/148380
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
