Support sparse binding

[Rua]

A long-standing missing feature. Some of the basic logic is currently there, but a lot more is needed. In particular:

• Buffer and image state tracking needs to track what memory is bound where at any given time. Not just for validation purposes, but also to keep the memory allocations alive.
• When submitting a command buffer, checks should be performed to ensure that all the accessed regions of buffers and images in that command buffer have memory bound to them. This needs to take into account sync ordering (sparse binds are not ordered unless synced with semaphores).
• How should sparse binding interact with subbuffers?
• How to deal with host access? Sparse memory is not contiguous, so the current read and write methods, which return a slice, don't work.