Make SkBitmap::CopyTo respect requested dst color type when bitmap is texture backed.

BUG=chromium:550559
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1576983002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1577393002
7 files changed