Introduce SkRGBA4f, templated on SkAlphaType

Most functionality is shared, but this lets us get type safety.
SkColor4f = SkRGBA4f<kUnpremul>
SkPMColor4f = SkRGBA4f<kPremul>

Change-Id: I27408565dc92e722f42a185cecbf7af15d1dde3f
Reviewed-on: https://skia-review.googlesource.com/156243
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
16 files changed