relax transform aliasing a bit

We can transform in place any time the source and destination
pixels are the same size.

Change-Id: Ib6a6ed1904954ec3429178b80321a62a1643d519
Reviewed-on: https://skia-review.googlesource.com/150861
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed