add bgra as 1st class format

This is a start to eliminating swap_rb as a stage.

I've just hit the main hot spots here.  Going to look into
the ~dozen other spots to see how they should work next.

Change-Id: I26fb46a042facf7bd6fff3b47c9fcee86d7142fd
Reviewed-on: https://skia-review.googlesource.com/20982
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
8 files changed