add configure option (disabled by default) to enable the 50ms delay
between VGA mode changes that DOSBox normally does by default. Hopefully
now when I do DOS demoscene captures, I don't get junk frames anymore
whenever mode changes are involved. It is an option you want to enable
in some cases, like when running weird demos like DoWhackaDo.

Also added code to better addref/release disk images on disk/CD
swapping, which fixes the segfault I observed running Windows 95 with
two ISO images attached.