backend-x11: Get rid of Display when initializing the GL renderer
This makes use of EGL_EXT_platform_xcb which will allow passing a xcb_connection_t as the native_display, rather than using a Diplay * as created by Xlib.
With this change we remove any Xlib portions we had in the nested x11 backend.
Signed-off-by: Marius Vlad marius.vlad@collabora.com