Fix scanline decoding of rare RLE bmps

This also exposed a bug in rewinding RLE bmps, which I have also
fixed in this CL.

This should fix testcase7.bmp on Gold.

The image that I am adding to resources is in the
public domain.

BUG=skia:4730
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1689953002

Review URL: https://codereview.chromium.org/1689953002
4 files changed