Add private grpixelconfigs for alpha_8 and alpha_half

Bug: skia:
Change-Id: I5191b6e045aea2a5af2b305b5972ad1e638a7ace
Reviewed-on: https://skia-review.googlesource.com/71763
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
14 files changed