don't support converting from alpha to non-alpha

BUG=skia:

Change-Id: Ia06bef6c0bfc03b5ca9f569c07e993da0bbd67c3
Reviewed-on: https://skia-review.googlesource.com/5288
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed