Add new SampleBlender callback to SkVMCodeGenerator.

This CL doesn't actually allow the callbacks to be invoked from SkSL
yet, but the callbacks now exist and are now threaded through the
various callsites which will need them.

Change-Id: I00f43ff94de9da8d93daf2e59885eea6f87c2e3e
Bug: skia:12257
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/431696
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed