Add kRG_1616 and kAlpha_16 SkColorTypes

This also switches GrColorType::kR_16 to kAlpha_16 to more closely match raster.

Bug: skia:9121
Change-Id: I03c6e6c52c90aa4223478c5ea6c8b2ed8558f677
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239930
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
37 files changed