go.image/bmp: support decoding 32-bit BMPs, as well as top-down BMPs.

Tested manually, from "dwebp -bmp" output. dwebp comes from
https://developers.google.com/speed/webp/download

LGTM=bsiegert
R=bsiegert
CC=golang-codereviews
https://golang.org/cl/148020043
1 file changed