tear down one-big-switch SkRasterPipeline backend

This was a pretty serious performance regression, especially on ARM.
Code size was wash, almost exactly 1:1.

While I like this one-big-switch approach in terms of maintainability
and platform obliviousness, I'm not willing to take such a big hit.
This'll just have to stay platform-aware.

(The old code is also easier to debug and profile, but that's peanuts.)

https://perf.skia.org/e/?begin=1546440652&end=1546459404&keys=X2ec5a42b0071f8848a5875f14400493d&xbaroffset=39028

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: Ia47fd25bbe4145468efd67cbddef7e6f92de3f02
Reviewed-on: https://skia-review.googlesource.com/c/180940
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
8 files changed