Refactor stroke tessellation shaders

Splits the instanced and tessellation implementations into their own
separate files and merges their common code into a shared base class.
Renames GrStrokeTessellateShader to GrStrokeShader.

Bug: skia:10419
Change-Id: Ia731509858e682a605ee65c9ced1fd163e4c03f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/409036
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
22 files changed