Do color space conversion in XfermodeImageFilter

BUG=skia:

Change-Id: I69c92506fca62e8711cece005f169ac05d26c872
Reviewed-on: https://skia-review.googlesource.com/6324
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed