omxil_vout: Honor BLOCK_FLAG_PREROLL

When an input buffer is flagged as preroll block, the generated picture shall
not be displayed. This is for example used with SEI recovery point messages in
H264 streams, to ensure only valid pictures are displayed.

Signed-off-by: Julian Scheel <julian@jusst.de>
libvlc: libvlc_TeletextChanged event type added

Signal the applications whenever the teletext spu changes.

Signed-off-by: Julian Scheel <julian@jusst.de>
teletext: Check spu-es instead of teletext-es.

toggle_teletext sets spu-es to the first subtitle track listed in teletext-es.
teletext-es itself is not set and hence returns -1 if more than one track is
available. So query for spu-es and also check for it to be unequal to -1, as
a spu-es of -1 causes a segfault.

Signed-off-by: Julian Scheel <julian@jusst.de>