funnel all constant colors through append_constant_color()

My next step is to change the uniform_color context to

    struct {
       float r,g,b,a;
       uint32_t rgba;
    };

so that it's trivial to load in both float and 8-bit pipelines.

Change-Id: If9bdde353ced3bf9eb0c63204b4770ed614ad16b
Reviewed-on: https://skia-review.googlesource.com/30481
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed