By passing NULL to `g_signal_new` instead of a marshaller, GLib will
actually internally optimize the signal (if the marshaller is available
in GLib itself) by also setting the valist marshaller. This makes the
signal emission a bit more performant than the regular marshalling,
which still needs to box into `GValue` and call libffi in case of a
generic marshaller.
Note that for custom marshallers, one would use
`g_signal_set_va_marshaller()` with the valist marshaller instead.
We have srt{client,server}{src,sink} elements in accordance to the
norm of the connection oriented protocols. However, SRT connection
mode can be changed by uri parameters so it requires an integrated
element to handle the parameters.
fix: #740
