start-capture (and probably stop-capture) are unsound

[pythonesque]

Description
Currently, wgpu-core (effectively) just calls start-capture on the raw underlying device without performing any checks.

From @kvark

we could document the safety requirements for start_capture as:

1. device should be a valid device that's not lost
2. no capture needs to be already in progress

Device loss is covered #1624. The start/stop capture thing is a separate issue that is probably more straightforward to fix.

[teoxoy]

57f8757 (#4163) added device validity checks for start/stop capture.

But we still don't validate if a capture in progress.