util-x11: Rework init_window interface to accept XVisualInfo

This is a much more correct way of doing things. Previously, we would select
a visual when creating the OpenGL context, but then use a default visual when
creating a window. This was fragile and would fail if the default visual was
not identical to what we had created.

Now, instead, we pass the selected XVisualInfo to our init_window interface
and call XCreateWindow instead of XCreateSimpleWindow. This guarantees that
the visuals match as required.