Add color space xform fallback to GrMorphologyEffect

Add color space xform fallback to GrMorphologyEffect

99% of the time (or maybe 100%?) morphology will trigger pad_image,
so the input texture will already be in the destimation color space.
If that doesn't happen, then just force the source to be converted,
which keeps the morphology effect and driver code simple.

BUG=skia:

Change-Id: I98876af4f9e9a5da031973213ed76349752ce68f
Reviewed-on: https://skia-review.googlesource.com/6388
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>