Move GrStrokeTessellator into its own header file

Simple refactor to make things reusable for the next non-hardware stroke
tessellator.

Change-Id: I0898b54a616e60f0475ac74cbd6f518e8696e0e5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/390078
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
10 files changed