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.