platforms/drm: kill the EglStreams backend

The proprietary NVidia driver now supports gbm, which vastly improves the user experience. For older devices that will not get gbm support dropping EglStreams will likely not have a big impact as it has several session breaking issues anyways.

By removing the backend a lot of logic can be simplified, most notably multi-gpu. In addition with introducing OutputLayer and making subclasses of it responsible for a bunch of the render backend stuff I think the simplifications will be very significant.

The only question is: should this wait for 5.24? In practice the wayland session is borked for most of our users right now anyways with the egl-wayland + Qt issue and the world hasn't ended, so I think it's safe to say that the amount of actual users of the EglStreams backend is very low.