Use a default fps instead of GST_CLOCK_TIME_NONE

If we use a GST_CLOCK_TIME_NONE for the duration, the frame will always
be part of the segment and thus always sent downstream. This can give
problems in case the video is being pushed, the sink does a preroll with
the wrong buffer and then waits for a FLUSH/PLAYING.
If the media has another stream (audio for example) which does a
segment clipping correctly we can have a blocking situation if an
audio stream buffer does not reach the audio sink and the video stream
is blocked because of a waiting for preroll
Add the xvid and divx variants for mpeg4 video

Looks like all the mpeg4 video decoders also accept the xvid and divx
variants. So we better add those caps too