fix(wgpu): invisible windows on X11 desktops with dGPU+iGPU

[mmstick]

Should fix these Reddit threads:

https://www.reddit.com/r/pop_os/comments/1dwwf56/cosmic_store_not_ready/
https://www.reddit.com/r/pop_os/comments/1dl5x27/cosmic_store_error_after_install/

Closes pop-os/libcosmic#512

[Drakulix]

Hmm, it would kinda be cleaner to read out the drm-device used by the xserver via dri, similar to how we handle the wayland case.

That way we don't make any assumptions, which gpu is used as the main gpu depending on the chassis type.

I could imagine this could cause similar issues otherwise when explicitly selecting discrete-mode on our laptops.

[Drakulix]

We could easily borrow some code from smithay to do that: https://github.com/Smithay/smithay/blob/master/src/backend/x11/mod.rs (mainly egl_init and dri3_init).

(Though we would want to use x11rb's xcb feature to use the raw pointers from the wgpu surface.)