Do software clip mask generation with worker threads

Also refactor the prepare callback stuff to share logic
between software path rendering and clip mask generation.

Bug: skia:
Change-Id: I0c56c6df8703eb59d2d49a4c3985bd4f5ef20f01
Reviewed-on: https://skia-review.googlesource.com/44421
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed