add srcover_bgra_8888

Chrome generally uses BGRA buffers, so srcover_rgba_8888 isn't really
doing them any good.  Probably a good idea to cover both kN32 options
any time we specialize like this?

There's one small diff, so I've lazily guarded this by
SK_LEGACY_LOWP_STAGES, which I want to rebaseline today anyway.

Change-Id: Ice672aa01a3fc83be0798580d6730a54df075478
Reviewed-on: https://skia-review.googlesource.com/63301
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
7 files changed