speed-up 4f version of modecolorfilter and modulate

~20% faster

1. Don't call color4f::premul on every invocation
2. Inline modulate as its own special case (used by shadows)

Bug: skia:
Change-Id: I49ca565f589b9a7caac88e95468da7f8c395804a
Reviewed-on: https://skia-review.googlesource.com/15613
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
2 files changed