Add basic support for kRGBA_10x6_SkColorType

This is four 10-bit channels, in a 64-bit word. Each channel
is stored in the 10 high bits of a 16-bit block, with 6
unused low bits.

Bug: b/283989374
Change-Id: I4ef829ac68df28e056abe280a9bd23e81fee589e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/750976
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
22 files changed