Implement sk_tool_utils::copy_to_g8(), used by gms

BUG=skia:

Change-Id: Ic5b2b07fb3f408edf1f2b982235424c22795fe50
Reviewed-on: https://skia-review.googlesource.com/7187
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
4 files changed