Rename append_uniform_color() to append_constant_color().

The color appended here is both uniform and constant, and it's the
constantness that makes this custom append method useful over just
append(SkRasterPipeline::uniform_color, ...).

Uniform colors that are not constant have to be loaded from the pointer
each time (the caller might have changed the color out-of-band), but
constant uniform colors can be analyzed once and implemented with
specalizations like black_color and white_color.

Change-Id: I3cfc00ccc578dd915367bca7113010557181224c
Reviewed-on: https://skia-review.googlesource.com/30560
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
6 files changed