Everything is now safely tucked inside each individual GLInterface.

This is effectively unused, as the window handles that we pass to the
GLInterface are window handles for the frame which isn't ever a real
toplevel window. Host_UpdateTitle is what actually sets the proper title
on the render window.

Now that MainNoGUI is properly architected and GLX doesn't need to
sometimes craft its own windows sometimes which we have to thread back
into MainNoGUI, we don't need to thread the window handle that GLX
creates at all.

This removes the reference to pass back here, and the g_pWindowHandle
always be the same as the window returned by Host_GetRenderHandle().

A future cleanup could remove g_pWindowHandle entirely.

Our existing code was relying on the GLX backend to create the GLX
window properly, and for the rest of the code to patch that up, sort
of. If we rely on Host_GetRenderHandle() returning a valid window, we
can do a lot better about this.

Create a simple window inside MainNoGUI to make this happen.

Move to one display. There's no reason to have two displays here -- the
comment stated that one should touch GLX and one should touch window
events, and that they should be touched from different threads, but the
current code wasn't this careful.

Just use one Display connection.

Also remove x, y, width and height from the GLInterface, since it's only
used in GLX, which no longer uses them