Implement radial gradient layout

Adds a radial gradient FP in the same vein as the prior linear gradient
FP. It updates SkRadialGradient to try and use the new system before
falling back to the original GPU gradient code.

Bug: skia:
Change-Id: I0739832beeae18d0a7178ada44014d7885478031
Reviewed-on: https://skia-review.googlesource.com/148803
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed