Respect canvas size and frame offset in webp decoder

BUG=skia:6185

Change-Id: Id543cb689a5e33b800ebbc18f4a234e78a4c4298
Reviewed-on: https://skia-review.googlesource.com/7800
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
5 files changed