[CM] snd infinite recursion

[Bill Schottstaedt]

Meino Cramer noticed that the gtk version of Snd 10.1 can get into an
infinite loop if you move the "zoom" slider on a somewhat large
file.  This bug was a side-effect of other changes that were trying to
prepare for Gtk 3 (they are hiding the gtk structs, which is a good idea,
but the replacement functions are subtly different in at least one case,
causing an unanticipated callback causing an infinite loop).  My fix
seems to make the slider jerky, but at least it isn't catatonic.