Add GalliumLLVM GrGLRenderer and fix alpha8 renderability

Bug: skia:
Change-Id: I20f41966ed140327e0a92618da8e4091e1de1ea8
Reviewed-on: https://skia-review.googlesource.com/64260
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed