Refactor RGBA/BGRA xform logic in SkCodecs

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4554

Change-Id: Ic9105a2806b915fc56b6810a80dd444561d0d959
Reviewed-on: https://skia-review.googlesource.com/4554
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
9 files changed