Multi GPU support on Wayland

• refactor udev to list all available GPUs
• refactor DrmBackend to use the outputs of all GPUs
• create a new OpenGLBackend that contains multiple other backends
• enable EglGbmBackend to copy buffers from other GPUs
• make Atomic Mode Setting work properly on multiple GPUs (it doesn't on my secondary GPU)
• enable EglStreamBackend to copy buffers from other GPUs (!1031 (merged))
• enable EglStreamBackend to render outputs for other GPUs

To complete the support:

• enable automatic detection of the NVidia proprietary driver
• fix VSync on secondary GPUs
• enable hotplugging of GPUs
• add fallback mode with dumb buffers