basic pass at gradients

This is looking pretty good.

Some egregious diffs, some invisible,
and a handful in-between I've investigated.

The egregious diffs look like bad shader caching.  I need to fix that,
but its brokenness is simply helpfully exposed here, not made any worse.
To repo, run: lumafilter, srcmode.

These in-between diffs all fall within the existing variance:
   crbug_938592               ok
   analytic_gradients         ok
   gradients_dup_color_stops  ok
Probably different geometry math, FMAs, reordering, etc.

I will follow up with radial etc,
and shallow gradients also demonstrate we're missing dither.

Change-Id: I5a046ee42accdf1faed50b3b65efb4a6787b5e04
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265821
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
7 files changed