Implement textured gradients

Also clones SkGradientBitmapCache into GrGradientBitmapCache in the
gpu/gradients folder. But after cleaning up the old gradient code,
SkGradientBitmapCache will go away and SkGradientShader will have no
reference to the bitmap cache or support for building bitmaps.

The "new" GrGradientBitmapCache has been updated to hide the thread
safety responsibilities and gradient bitmap generation code that had
originally been a part of SkGradientShader.

Bug: skia:
Change-Id: Ida134c6437c866439fac44fa453d09a6a11549e7
Reviewed-on: https://skia-review.googlesource.com/150917
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed