Staging buffer sizes should be per-queue

[sherief]

A per-process one-size-fits-all might work for games on Xbox, but working on a high-end PC content creation tool this is just not working for me. 4K uncompressed textures (development build, content creation, background async compression while live reloading etc) need a large staging buffer size, while during sculpting geometry I can live with much smaller sizes and I want the VA / BAR address space.