videoaggregator: don't do caps processing that is not overridable

videoaggregator: don't do caps processing that is not overridable

Allows the subclass to completely override the chosen src caps.

This is needed as videoaggregator generally has no idea exactly
what operation is being performed.

- Adds a fixate_caps vfunc for fixation
- Merges gst_video_aggregator_update_converters() into
  gst_videoaggregator_update_src_caps() as we need some of its info
  for proper caps handling.
- Pass the downstream caps to the update_caps vfunc

https://bugzilla.gnome.org/show_bug.cgi?id=756207