add Start/EndFragmentProcessor to DSLWriter

Change-Id: I6f9d414d55b3ba64c3f67f3a590f6b7b5642062a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/364599
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
5 files changed