Dawn: implement staging buffer manager.

Use managed staging buffers for texture uploads, uniforms and buffers.

Change-Id: I063707c160236725d27a1d1bfb197d3096a07b34
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238120
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Greg Daniel <egdaniel@google.com>
11 files changed