This refactors from_half() and to_half() a bit, totally
reimplementing the non-hardware cases to be more clearly correct.
CQ_INCLUDE_TRYBOTS=skia.primary:Test-Android-Clang-PixelC-CPU-TegraX1-arm64-Release-Android,Test-Android-Clang-Ci20-CPU-IngenicJZ4780-mipsel-Release-Android,Test-Android-Clang-Nexus10-CPU-Exynos5250-arm-Release-Android,Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Release,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86-Debug,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug
Change-Id: I439463cf90935c5e8fe2369cbcf45e07f3af62c7
Reviewed-on: https://skia-review.googlesource.com/13921
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
