write_buffer on another thread can block get_current_texture for an extra frame

[danielkeller]

I'm using wgpu on macos/metal, and when I call write_buffer on a different thread from get_current_texture, about 20% of the calls to write_buffer will block both calls for an extra ~16 ms. Here's a screenshot of puffin viewer showing the problem:

It looks like write_buffer might be taking a lock at just the wrong moment and causing get_current_texture to miss the screen refresh, and locking up both threads until the next screen refresh.

[kvark]

Similarly to #2395, this is blocked on #2272

[cwfitzgerald]

Obsoleted by #2710