GSTREAMER SIDE:
 - find a way to dynamically link the pipeline to not stop playing
   while switching from the effect little preview to full size video
   output.
 - this can't obviously be done with multiple pipelines since the
   webcam device is locked.

 IDEA: source (full size) --> tee1
                    ___________|_____
                   /                 \
        filter-1 --> bigsink-1      tee2 --> videoscale (downscale)
        filter-2 (unlinked?)                     |
         ...                                 videorate (downrate)
        filter-n (unlinked?)                     |
                                                 +--> filter-1 --> sink-1
                                                 +--> filter-2 --> sink-2
                                                  ...
                                                 +--> filter-n --> sink-n

sink-0 is the output window of the full size image.
sink-1..n are the output windows of the effects previews scaled down to
optimize cpu/gpu usage in the effect selector
Outstanding issues: 
- we need a way to selectively pause (unlikely viable) or dinamically
  link-unlink tee branches never stopping the source (the webcam is slow
  to startup and it would be quite ugly in the whole user experience).
- do we need to prelink all the bins at both reduced and full size?
- part-block.txt could probably a useful read:
  http://cgit.freedesktop.org/gstreamer/gstreamer/tree/docs/design/part-block.txt

CLUTTER SIDE:
 - a custom container or layout manager for the effect
   selector. Should be able to handle stage resizing.
 - come up with a nice way to organize several effects: if we have 20
   different effects I suppose there is a little overhead with having
   all displayed and running at the same time. Possibile solution: a
   multipage grid based selector; again, what should we do with effect
   previews not currently displayed, pause them (unlikely)? unlink
   them (to investigate)?
 - find or create from scratch a throbber actor to draw while
   pipelines are loading.