allow the caller to specified raster-surface rowbytes.
along the way, simplify how we copy the surface's bitmap

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1643873002

Review URL: https://codereview.chromium.org/1643873002
3 files changed