Add Ganesh support for RGBA_16161616 and RG_half

Change-Id: Ia424ff719c5fb8d1842a909ade09ad6f5f78f21f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/219998
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
22 files changed