Support sRGB 565.

It looks like I'm not going to be able to avoid supporting sRGB G8, I8, 565, 4444, 8888.
(A8 and F16 will always be linear.)  This fixes 565, and lays out the rest of the accum_*.

I did a little reorganization to keep things in ascending bit depth, just for sanity.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5145
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: Ib0508e5a4ee1bab2044a76bcabc367841d634cd2
Reviewed-on: https://skia-review.googlesource.com/5145
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed