fix CMYK alpha channel

Change-Id: I803d050d5ff2e4c841df31585f13016f6b178ae3
Reviewed-on: https://skia-review.googlesource.com/c/163161
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
9 files changed