rtpdec: Fix the calculation of expected number of packets

The base_seq variable is set to first_seq - 1 (in
rtp_init_sequence), so no + 1 is needed here.

This avoids reporting 1 lost packet from the start.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec: Calculate and report packet reception jitter

This brings back some code that was added originally in 4a6cc061
but never was used, and was removed as unused in 4cc843fa. The
code is updated to actually work and is tested to return sane
values.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec: Send a valid "delay since SR" value in the RTCP RR packets

Previously, we always signalled a zero time since the last RTCP
SR, which is dubious.

The code also suggested that this would be the difference in
RTP NTP time units (32.32 fixed point), while it actually is
in in 1/65536 second units. (RFC 3550 section 6.4.1)

Signed-off-by: Martin Storsjö <martin@martin.st>