try not zeroing registers in start_pipeline

Generally stages take care of state setup themselves, either with
seed_shader, constant_color, a load, etc.  I think these zeros may
be unnecessarily cautious.

This can't make anything draw more correctly, but it could make things
   - draw wrong
   - draw more slowly
   - draw more quickly
so it's an interesting thing to try and keep an eye on.

Change-Id: I7e5ea3cd79e55a65e1dbd214601e147ba3815b87
Reviewed-on: https://skia-review.googlesource.com/20976
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed