Make SkWEBPImageEncoder use transform_scanline_procs

This refactor will make it simple to add F16 and
GammaCorrect support to the webp encoder.

BUG=skia:

Change-Id: I379ba2ff0b44d865847877322e86d0c138e727c8
Reviewed-on: https://skia-review.googlesource.com/7356
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
4 files changed