port encode_bitmap_for_png() to SkRasterPipeline

This logically shouldn't change anything, but it does a bit:
   - no diffs in 565
   - sRGB and 8888 show minor diffs, only when unpremul is non-trivial
   - f16 shows minor diffs all over, and major ones in shallow gradients

I think the shallow gradient diffs make sense. F16's high-precision is
being quantized down to a much narrower range (something like 10-40
values), so where the lines are drawn between sRGB bits will come down
to all sorts of things, especially rounding.

Change-Id: I156b7c613b73d6d6089221e61d0c529798ac1f9f
Reviewed-on: https://skia-review.googlesource.com/19449
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

0 files changed, 0 insertions, 0 deletions