mpeg4videoparse: do not lose DISCONT flag if buffer isn't pushed

mpeg4videoparse might not push buffers while parsing. If those buffers
contain the DISCONT flag, it gets lost and downstream won't get any
buffer with the flag.

Fix it by adding the DISCONT to the next pushed buffer.
This makes backwards playback work.
dashdemux: fix seeking when the mpd uses a segment template

Remove the dashdemux seeking function to use the one implemented
in mpdparser as it is more complete. This also makes dashdemux not
crash when seeking on streams that use segment templates.
dashdemux: remove unnecessary variable

Instead of using 2 indexes that were actually the same, use only
one. Saves a variable.
aiffparse: misc code cleanups

Fix the launch examples for 1.0. Turn more logging into _OBJECT variants.
configure: check for good enough dvb kernel headers

Recent changes are using new dvb api. Don't built the plugin, if we can't.
Fixes #721869