tsdemux: Push GAP events *after* deactivating old programs

tsdemux: Push GAP events *after* deactivating old programs

The order in which program switch must happen is:
1) drain all data on old pads (but don't push EOS)
2) add new pads (but don't push any data on them)
3) Push EOS and remove old pads
4) Start pushing data on new pads

There was one caveat in this implementation, which is that when
we activate a sparse pad (step 2) we would push a GAP event. The problem
is that, while being an event, it is actually *data*.

We therefore need to make sure pushing those GAP event is done at the step
we start pushing data.

https://bugzilla.gnome.org/show_bug.cgi?id=750402