pngparse: Proxy upstream framerate on srcpad if avalaible

If the user sets it himself with capsfilter, the information should be
kept and transmitted downstream.
mssdemux: also consider /manifest

The spec says /Manifest, but /manifest is easily found on some
services, so handle that, too.

https://bugzilla.gnome.org/show_bug.cgi?id=710223
mssdemux: do not try to deinit the downloadrate while it is used

This can cause an assertion or deadlocks.
srtpdec: Pass is_rtcp to setcaps function
srtpenc: Set the first session flag to false once one is create

Otherwise it always thinks it's the first session.

https://bugzilla.gnome.org/show_bug.cgi?id=711495
vtenc: Use correct strides, etc from the GstVideoFrame

https://bugzilla.gnome.org/show_bug.cgi?id=706211
mpeg4videoparse: accept VideoObjectLayer as start of frame and config

... which it deserves as much as VideoObject.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=667564
dvdspu: pop new packet when invalid next command block

... to prevent staying stuck on current command block and busy looping.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=712219
dvdspu: Don't unmap an unreffed buffer

gst_dvd_spu_finish_spu_buf drops the reference to the current
buffer, so make sure to call gst_buffer_unmap before it.
dvdspu: Handle vobsub packets smaller than 4 bytes

In particular, handle the case of an empty packet with size 0 bytes.

Partially fixes: #712643
mpegvideoparse: look beyond start code before leaping

In case more data than a start code alone is needed to decide whether
it ends a frame, arrange for more input data and decide when available.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=711627
mpgegdemux: do not use pushed buffer in debug message
mpegvideoparse: tweak extension handling