add a callback stage to SkRasterPipeline

add a callback stage to SkRasterPipeline

This lets us temporarily escape to piece of code outside
SkRasterPipeline.  We should be able to use this to replace
   - parametric_{r,g,b,a}
   - table_{r,g,b,a}
   - color_lookup_table
   - shader_adapter*

* We want to obsolete shader_adapter for other reasons anyway,
but we _could_ replace it with this if we want to.

Change-Id: I42b657b3c19c679796ed1876856cae0c8471307e
Reviewed-on: https://skia-review.googlesource.com/12102
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>