Reland "Respect full precision for RGB16 PNGs" (part 3)

Reland "Respect full precision for RGB16 PNGs" (part 3)

This lands the rest of the original CL.
It fixes some flawed logic in SkSwizzler handling Gray8
images.

Original CL:
https://skia-review.googlesource.com/c/7085/

BUG=skia:

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-MSAN,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD,Build-Ubuntu-Clang-x86_64-Release-Fast

Change-Id: Ie2f0c545ea474f1872f284dfb286987b6eadf03d
Reviewed-on: https://skia-review.googlesource.com/7320
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>