Bump stored lowp uniform color to 16-bit storage.

This makes loading into 16-bit channels more natural in _lowp.cpp.

Change-Id: I1ed393873654060ef52f4632d670465528006bbd
Reviewed-on: https://skia-review.googlesource.com/47261
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed