This allows async buffer mapping to function when a renderpass
is also submitted on the queue. The state transition and fencing
still needs work, however.
These callbacks are executed outside of the mapping function and could
previously reference data that had been dropped. This fixes the
soundness issues described in #95, but not the buggy mapping
behavior.
Adds preliminary transitioning of buffers to mapped state.
Adds buffer unmapping to the cube sample.
Modifies wgpu_queue_submit to not hold a write lock on the device during callbacks.
