The GLX and EGL winsys backends had a check for when onscreen==NULL
in which case they would instead try to bind the dummy surface. This
wouldn't work however because it would have already crashed by that
point when it tried to get the Cogl context out of the onscreen. The
function needs a bit of refactoring before it could support this but
presumably nothing is relying on this anyway because it wouldn't work
so for now we can just remove it.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
a72a2c99fe