tsdemux: Implement keyframe scanning for MPEG2/4

Submitted by Sebastian Dröge @slomo

Link to original bug (#757459)

Description

We currently implement this for h264 only, while it's much simpler for MPEG2/4. I once implemented that in MXF, code can be found here:
http://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/gst/mxf/mxfmpeg.c#n436
http://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/gst/mxf/mxfmpeg.c#n487