backend-x11: Get rid of Display when initializing the GL renderer

Marius Vlad requested to merge mvlad/weston:wip/mvlad/xcb-native-display into main

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

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