backends/x11: Use DRI3 directly

At the moment, the buffers for wsi are allocated implicitly by the EGL implementation, which is fine for "normal" use cases. But we start hitting the ceiling the moment we need to something more advanced. For example the EGL backend creates a dummy fbo object wrapping the default framebuffer, meaning that we cannot pass it to qtquick (because it can use its own opengl context).

Another reason for using explicit buffers is that it lets us to clean up some output related abstractions.