... which is upon receiving new data with different PTS spec-wise,
or optionally upon each packet of subtitle data if desired by property.
Fixes https://bugzilla.gnome.org/show_bug.cgi?id=669310
Conflicts:
gst/dvbsuboverlay/gstdvbsuboverlay.c
Push incoming subtitle pages in a FIFO queue (pending_subtitles)
and dequeue the head when it's time to show it (when video running
time reaches the subtitle page running time).
Keep the subtitle page, that is supposed to be blended on top of video
currently, in a separate object variable (current_subtitle). As a
next step we can then pre-render current_subtitle to a better to blend
format.
