[graphite] Add VulkanBuffer map and unmap methods


Progress defining a map() call

Change-Id: Idba4ac9aaf3db4bfd4d2453611fc888b8ac2e76c
Bug: b/237108200
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/614056
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
8 files changed