add SkColorTypeIsNormalized()

This centralizes the logic for whether a color type is normalized.

The change in SkRasterPipelineBlitter is minor, only now properly
treating kR16G16_float_SkColorType as unnormalized.

The change in SkColorSpaceXformSteps is more extreme given the way
it had been written, with all the newer color types now correct.

I think I'm making the right call on kA16_float_SkColorType?

Are there equivalent sites to update in Ganesh?

Change-Id: I32a40b31b86c5fde0dea2528122a4deda91c5545
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253668
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed