clean up low-hanging swap_rb

There are two remaining swap_rb uses that both look non-trivial to
replace:
   - sampling out of index8 when the color table is bgra
   - table transforms on bgra inputs in SkColorSpaceXform

I don't think it's a big deal to just leave swap_rb around,
just a little sad.

Change-Id: I3d30200cf867cbf37d6f86572b1574d3e22e3490
Reviewed-on: https://skia-review.googlesource.com/21040
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
10 files changed