rough clamped tracking in SkRasterPipeline

This should make srgb_color_filter draw correctly in software.
Previously the Rec2020 block would overflow.

Change-Id: Ied4516728039e54214886d55bba92662beee9a26
Reviewed-on: https://skia-review.googlesource.com/26562
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed