Change the interface for fences to accept mutable references. This will
actualy help the Metal backend more than dx12 (avoiding interior
mutability) but more accurately captures intent and matches gfx-hal.
Adds a new "mux" module which can have multiple backends. As of this
commit, it's not wired up at all, but the functionality should be
reasonably complete.
Minor tweaks to the backend trait to accommodate this, mostly changing
Fence and Semaphore to references so they don't need to be Copy.
Part of the work toward #95
These function, but can use some work.
First, the buffer situation is worse than it should be. It should be
possible to create a single readback buffer rather then copy from
gpu-local to host-coherent.
Second, the command buffer `finish_timestamps` call doesn't correlate to
anything in Vulkan, so needs plumbing up through the hub in one form or
other when that happens. I'm inclined to make it ergonomic by doing a
bit of resource tracking that will trigger the appropriate call (and
subsequent host barrier) in the `finish` method on the command buffer.
Create compute pipelines from shader source and descriptor sets. This
gets it to the point where it can run the collatz example.
Still WIP and with rough edges, of course.
Chipping away at the dx12 backend. This should more or less do the
signalling to the CPU that the command buffer is done (ie wire up the
fence). It also creates buffer objects.
