get familiar with SkBlitMask_D32.cpp

  - some refactoring
  - remove SK_RESTRICT
  - try to remove SK_SUPPORT_LEGACY_A8_MASKBLITTER

Change-Id: I3a270fa2423a66f6e49e5f4f89593a27a9a26e9f
Reviewed-on: https://skia-review.googlesource.com/c/170061
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed