PatchWriter vertex allocation is templated

This is the last step to allow graphite to drop in a different
VertexWriter mechanism that works off DrawBufferManager/DrawWriter
isntead of GrVertexChunkArrayBuilder for Ganesh.

Bug: skia:13012
Change-Id: I986b102d951f9ea133e35a30376f775992d484e5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/518938
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
6 files changed