ges: Properly position video sources in the scene by default

ges: Properly position video sources in the scene by default

We try to do our best to have the video frames scaled the best way
to fill most space on the final frames, keeping aspect ratio. The user
can later on rescale or move the sources as usual but it makes the
default behaviour a better and more natural especially now that we
set default restriction caps to the video tracks.

And fix the unit test to take that change into account