SkBitmap::ComputeIsOpaque and SkBitmap::eraseColor support F16

Also add a unit test.

Change-Id: I9b6635ce9dd504788ca36b3246eaac2b37c2f3a6
Reviewed-on: https://skia-review.googlesource.com/5443
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed