Restructure path tessellation shaders

Merges the GrStrokePathShader and GrFillPathShader classes together.
Creates a new base class called GrPathTessellationShader. Now they all
have a uniform color and can all be draw to stencil and color both.
This is necessary cleanup in general, but will also allow us to create
a convex tessellation op that bypasses the stencil buffer.

Bug: skia:10419
Change-Id: Ifc492c94d3de044a36bd9ea95b1d5aa22e007905
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413696
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
23 files changed