SkMesh buffer updates work on GPUs without transfer buffers.

Adds a task that takes a copy of the update data and updates the buffer.

Bug: skia:12720
Change-Id: Idb56b2eb4d8ac4a1e5b526549368e605a06d22c9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/569981
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
9 files changed