Merge GrStrokePatchBuilder into GrStrokeTessellateOp

There's really no reason for these classes to be separate at this
point. Also extracts a "GrStrokeOp" base class that has the
functionality that will be shared with indirect stroking.

Bug: skia:10419
Change-Id: I960d5e6d64f0814ccb4a3852bc627af2b8082a1f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/331860
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
8 files changed