Add support for Alpha8, Gray8, and 4444 pixel formats

Gray 8 is particularly special: It's always computed as luminance (Y),
transformed by the destination transfer function. When invoked with no
color profiles, src and dst are assumed to be sRGB.

Change-Id: I2218924d2f43e1e81b3aa48db55398acea4a7a41
Reviewed-on: https://skia-review.googlesource.com/136604
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed