videoaggregator: handle gap buffers properly

videoaggregator: handle gap buffers properly

This simply implies not trying to "prepare" those buffers,
as mapping an empty buffer to a video frame does not make
much sense.

This also adds a simple test in compositor that performs
some trivial checking of the handling of gap events, the test
was not failing before, but an error was logged, this is
no longer the case.

Fixes #717