Refactor SkBmpCodec

I started working on indicating the native encoded formats
and things got really complicated for bmp.  I think starting
with this refactor may help a little, and I also think that
this is a good change by itself.

BUG=skia:4133
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1820283002

Review URL: https://codereview.chromium.org/1820283002
2 files changed