Add preserves premul and modulate optimization to compose fragment processors.

Fixes out of range colors produced by matrix convolution and dither effects. Adds modulate optimization to matrix convolution.

Change-Id: I8424250a52e864f4b5feaf4474293695c26039d8
Reviewed-on: https://skia-review.googlesource.com/8351
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed