add HALF support to PixelFormat

This is somewhat speculative; the newly supported F16 alpha and F16
red-green formats are not widely used or tested.  This will become
better covered soon by F16 RGBA when I add support for 64-bit pixels.

Change-Id: Ie1260e2bb5e9c057064e3133937ff6910ab84269
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303297
Commit-Queue: Herb Derby <herb@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed