This will help users determine whether the problem is:
* a backend is not statically enabled
* a backend is not dynamically enabled
* no drivers or physical adapters are present for that backend
(further distinction would be useful here)
* no adapters met the required criteria
There are deficiencies in the reporting of WebGPU vs. WebGL support.
Those would best be fixed by also fixing the mutual exclusion of those
backends.
This allows `wgpu` to not unconditionally depend on `std::path::Path`.
It’s also, in my opinion, more user-friendly, because the feature which
most users will not use (and is not currently functional) is now a
defaultable struct field instead of a required parameter.
The disadvantage is that `wgpu-types` now has to know about tracing.
According to winit docs, pre_present_notify() should be called right
before calling present().
This actually prevents some issues on Wayland, like freezing the whole
application when the window is not visible (ask me how I know).
