Created attachment 17453 [details] [review]
patch

_surfaces_compatible in cairo-xlib-surface returns true for surfaces with
different xrender_format when each has the same depth and no (NULL) visual.

Common picture formats will not have the same depth, but
it is possible to create a surface with a non-standard xrender_format having
the same depth as another xrender_format with
cairo_xlib_surface_create_with_xrender_format.

Both cairo_xlib_surface_create_with_xrender_format and
_cairo_xlib_surface_create_similar_with_format create surfaces with no visual.

The same issue exists in the xcb backend.