Image filters: fix filtering of transparent black in CFIF.

SkColorFilterImageFilter has a codepath to filter transparent black
regions outside of the input image. However, it was treating the
exterior as opaque black, rather than trasparent black.

Note: the results of imagefiltercropexpand GM will change, as it
was tweaked a bit to show this bug.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1978363002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/1978363002
2 files changed