fix lowp gradients

They were not keeping their colors in gamut.  This is most visible in
the completely garbage gradients in --config narrow.

This fixes most of the egregious problems with --config narrow.

Change-Id: I488f6fd083207ce7a880fb0afe0eb778c427a929
Reviewed-on: https://skia-review.googlesource.com/152585
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed