EGL and gbm device ordering woes

[i509VCB]

While working on the x11 backend in my own compositor some time ago, I realized that an EGLContext created using a display made from a gbm device will segfault on drop if the gbm device is dropped before the context.

Manipulating drop order so that everything egl related is dropped first does prevent such a segfault but it is quite a weak workaround.

@Drakulix you mentioned some time ago you'd like to fix such an issue, so here is an issue on github.