When using a renderer, the filtered demux will report the time currently
displayed by the renderer. If slaves synchronize to this time, SPU or other ES
tracks will always come too late. To fix this issue, we fetch the time on the
original demux (that will report the time being buffered).

This could be removed in VLC 4.0 in favor of a new demux control, like
DEMUX_GET_BUFFERED_TIME.