Add creation-time POD memory pool for GrOps

This CL begins pulling some of the work forward into onPrePrepare.

Change-Id: If049e0662db51b465b8b82aafebeef2323bddfd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249802
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed