Switch gradients from GrColor4f to SkPMColor4f

This is the most misleading use-case, because these values are either
premul or unpremul depending on the gradient's flags. It turns out that
we only need to "lie" and alias in one spot, though, so I think this is
better than adding an entirely new kUnknown_SkAlphaType specialization
of SkRGBA4f.

Bug: skia:
Change-Id: I4b7e280355219d439f50a7fe54230ea17715abc3
Reviewed-on: https://skia-review.googlesource.com/c/162745
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed