Added configurable vdpau output surface display queue length.
For smooth operation of frame displaying on platforms with less
(and in preparation for a new frame grabbing extension) the vdpau
display queue length,
which is actually fixed at size 2, could now be increased by
configuration. There is a new
configuration parameter 'video.output.vdpau_display_queue_length' with
default value of 2.
Maximum queue length is 8. Increasing this parameter by 1 should almost