isomp4/qtmux: Write correct file duration when gaps exist.

When writing out a trak with an edit list, make sure the
overall file duration is also updated to reflect the
lengthening of the stream.

Add some more debug to qtdemux to warn about streams that
are longer than the file and get truncated.
rtph264pay: append packetization mode parameter to SDP

Append packetization-mode parameter to SDP description.
Packetization mode signals the properties of an RTP payload type.

https://bugzilla.gnome.org/show_bug.cgi?id=733556
rtpmux: Unref pad template caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734473
navseek: Unref peer pad after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734476
imagefreeze: Unref pad template caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734475
videomixer: Unref allowed caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734474
rtph263ppay: Unref pad template caps after use

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734435
tests: rtpgstpay/-depay: add new unit test

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719587
rtpgstpay: delay event until next buffer, to get a timestamp

Previously events, e.g. tag or custom events, would be flushed out of
rtpgstpay's internal adapter before the first buffer arrived. The events
are not timestamped and so the output buffers containing the RTP packets
would not be timestamped either. Now, these events are delayed until the
first buffer has arrived. Moreover they will get the same timestamp as
the first buffer.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719587