add some lowp gradient stages

I was originally going to add these to help test a lowp dither, but
after looking at diffs I don't think lowp dither is a good idea.

Non-dithered lowp gradients look fine to me so far.

I'd have done conics, but they scare me.

Change-Id: I8f5e75aec726983186214845ca38cfa0d54496b3
Reviewed-on: https://skia-review.googlesource.com/66460
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
8 files changed