Fix bugs in SkBmpCodec

The decode should not depend on the requested alpha type.

These were exposed by:
https://codereview.chromium.org/1641273003

This should cause the number of untriaged images in Gold
to go to zero.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1663303002

Review URL: https://codereview.chromium.org/1663303002
2 files changed