Extract some GrPathTessellatoOp logic into virtual helper classes

This reduces complexity and will also allow us to add new ops that
take advantage of this same core logic.

Bug: skia:10419
Change-Id: I4ec8717a6d9510dea967d11467eeea0b5b7c7f4c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/354296
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
8 files changed