Instead, use the display's "leader surface" when no surface is required,
because we have it lying around.
Really, we want to use EGL_NO_SURFACE, but if that's not supported...
Instead of going via GdkVisual, doing a preselection and letting the GL
initialization improve it, let the GL initialization pick an X Visual
directly using X Visual code directly.
The code should select the same visuals as before as it tries to apply
the same logic, but it's a rewrite, so I expect I messed something up.
Avoids having to use private data, though the benefit is somewhat
limited as we still have to put the destructor in the egl code and can't
just put it in gdk_surface_x11_finalize().
We only have one config, because we use the same Visual everywhere.
Store this config in the GdkDisplayX11 struct for easy access.
Also do this on initialize, because if creating the config fails, we
want to switch to GLX instead of failing to do GL at all.
This also simplifies a lot of code as we can share Visual, Colormap, etc
across surfaces.
There's no need to use g_object_set_data() for it.
We can also stop caching it elsewhere because we know the display has
it.
And finally, we can remove the display->have_egl boolean and use
display->egl_display != NULL instead. We initialize the display at
startup, so that variable is the perfect indicator.
