Implement onSkipScanlines() for bmp and wbmp

TBR=reed@google.com
BUG=skia:4270
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1691083002

Review URL: https://codereview.chromium.org/1691083002
11 files changed