[ganesh] Add staging buffer manager to GL for SkMesh buffer updates

Bug: skia:12720
Change-Id: I69d46a4cbce881b61be768622daaacb13c78fa56
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/572252
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed