This is an instance of the bug where we run into the kernel-imposed limit of
64K mmaps, see also bug 94726. They're not necessarily GL buffer objects, as
internal buffers can contribute as well. In any case, this is non-trivial to
fix.

It seems like reducing the graphics quality might work around the problem in
the meantime. There is also a potential workaround-patch attached to bug 94726,
but it doesn't seem to actually work around the problem here.

Thanks Edwin for giving me access to a live game where this bug occurs, and
thanks Moritz for reporting the bug in the first place!