Have SkRasterPipelineBlitter take over for 565 when it can.

Have SkRasterPipelineBlitter take over for 565 when it can.

This is an experiment / demo to have our 565 backend fold into
SkRasterPipelineBlitter as it grows more powerful.  I plan to follow up with
the same for the other 8888 format.

Blur mask filters look significantly different (better) after this change.
We keep the full 13-14-13 bits of precision for mask blits, where the old code
uses 11-11-10 bit intermediates.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2172343002

Review-Url: https://codereview.chromium.org/2172343002