If an mp4 URL was set in the parameters, use it

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Support on-the-fly conversion to an .mp4

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make it work with current avconv

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make it work on an iPad

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>