bail out of ChooseBitmapXferProc with a color space

This fixes the paint-color parts of the gamut GM,
and a handful of other similar problems.

We'll still memset in the blitter, just after we get in the right gamut.

BUG=skia:6679

Change-Id: I2b16259ad689c63ae304fb600802acf9125b8c25
Reviewed-on: https://skia-review.googlesource.com/17922
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed