Add branching ops to Raster Pipeline.

This is based on the technique shown in http://review.skia.org/566561
but has been updated to latest code (no more void*s). Also, this
version avoids redeclaring the entire STAGE macro from scratch.

Change-Id: I731271d2cce89f48ee8b12c69ff4b4cd66d0110d
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/612636
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed