Retry, Bump stored lowp uniform color to 16-bit storage.

This makes loading into 16-bit channels more natural in _lowp.cpp.
Update a unit test to stop using out-of-range "colors".

Change-Id: I494687aac87948b60a40de447aa1527cf7167b2d
Cq-Include-Trybots: skia.primary:Test-Debian9-Clang-GCE-CPU-AVX2-x86_64-Release-UBSAN_float_cast_overflow
Reviewed-on: https://skia-review.googlesource.com/47580
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed