Dither copies when decreasing precision below 32-bit.

Same GM diffs as the last attempt in 8888.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Android-Clang-PixelC-CPU-TegraX1-arm64-Release-Android,Test-Win2k8-MSVC-GCE-CPU-AVX2-x86-Release,Test-Android-Clang-Nexus10-CPU-Exynos5250-arm-Release-Android

BUG=chromium:720105

Still looking at why readpixels and readpixelspicture are so weird
(lots of black) when drawing into sRGB 8888.  Will follow up.

Change-Id: I223e3b74e567aea1acaffa8db6b24fbf22d98c97
Reviewed-on: https://skia-review.googlesource.com/17443
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
1 file changed