Switch GrMeshDrawOp::Target to be the stand alone GrMeshDrawTarget class

The Tessellator classes will survive in the NGA and they use
GrMeshDrawOp::Target - but GrMeshDrawOp will not survive.

This is a prelude to making all the remaining GrOp-derived classes OGA-only.

Bug: skia:11837
Change-Id: I62dc92e5f42d672342113f12dcedf3435fab993f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419198
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
56 files changed