vk_scheduler: Allow command submission on worker thread

This changes how Scheduler::Flush works. It queues the current command
buffer to be sent to the GPU but does not do it immediately. The Vulkan
worker thread takes care of that. Users will have to use
Scheduler::Flush + Scheduler::WaitWorker to get the previous behavior.

Scheduler::Finish is unchanged.

To avoid waiting on work never queued, Scheduler::Wait sends the current
command buffer if that's what the caller wants to wait.