Reland "Respect full precision for RGB16 PNGs" (part 2)

This lands all the new xform hooks but no change to src/codec.
So the new decode features are turned off.

I'm relanding this in pieces to try to bisect a
strange MSAN error.

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: I451a2a29c73ca475e9e7a5ded58d4948d6b8be19
Reviewed-on: https://skia-review.googlesource.com/7277
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
8 files changed