Reland "Refactor trimming logic for read/writePixels()"

Original CL: https://skia-review.googlesource.com/c/7326/

(1) Move trimming logic into Bitmap/Pixmap level for
    raster.  Everything goes through here, so we'll
    only do the work once.
(2) This means it also goes to GPU level.
(3) Always use SkReadPixelsRec rather than inlining
    the logic.
(4) Create an SkWritePixelsRec to encapsulate write
    trimming.
(5) Disabled kIndex8 as a dst - always.

CQ_INCLUDE_TRYBOTS=skia.primary:Perf-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug

BUG=skia:6021

Change-Id: I25a964e3c610c4e36d195a255e2150657baec649
Reviewed-on: https://skia-review.googlesource.com/7404
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
12 files changed