multiqueue: do not reduce single queue below current level

multiqueue: do not reduce single queue below current level

When the single queue size was just bumped by 1 to allow more buffers to
be added, the buffers limit could be reduced to the current level when
setting the max-size-buffers property. This would result in a stall
since the queue would not grow anymore at this point.

Prevent this by not reducing a single queue size below the current
number of buffers + 1.

https://bugzilla.gnome.org/show_bug.cgi?id=712597