Metal backend's `wgpu_hal::Device::wait` implementation polls instead of waiting

[jimblandy]

In wgpu_hal, <metal::Device as Device>::wait checks MTLCommandBuffer's status every 1ms until it is Completed or the timeout is reached. This is very mid (if that's not a contradiction in terms).

There is no "wait with timeout" operation in Metal, but there is an addCompletedHandler method on MTLCommandBuffer, which is called "on an undefined thread". wgpu_hal::metal::Device could have a shared condition variable protecting the fence value, Queue::submit could register an addCompletedHandler callback to advance the fence value, and Device::wait could use std::sync::CondVar::wait_timeout to wait for a given value.