some basic speed ups for SkRasterPipeline::append()

The new bench demos the speedup:
    SkRasterPipelineReuse_…
    …full 1x  …some 1.8x  …none 5.22x

Change-Id: I5e51fb4316ae04558710ce62560850584ccb4aea
Reviewed-on: https://skia-review.googlesource.com/17449
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed