SkRasterPipeline: simplify impl and remove need to rewire stages

This builds the stages correctly wired from the get-go.  With a little clever
setup, we can also design around the previous error cases like having no stages
or pipelines that call st->next() off the end.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2149443002

Review-Url: https://codereview.chromium.org/2149443002
3 files changed