The (first) shared screen element will take up to 2/3 of the
matrix's width and 100% of its height. Other publishers (including other
shared screens) will fill the rest.

Also fixes small bug with too-big subscribers list height in some cases.

Future scope:

1. Display the shared screen publisher inside the shared screen tile.
2. Add a button to increase size of any publisher tile. Still, I think there should be only one big tile at a time. Anything more sophisticated requires a much harder algorithm.

./phpunit --group=openvidu