qtmux port to GstAggregator caused splitmux test failures

In 1.16, qtmux refuses incompatible caps changes on a running mux, and that prompts splitmuxsink to start a new output fragment.

Since the port to GstAggregator, qtmux accepts any caps event it receives. That happens, because GstAggregator accepts the caps event, returns TRUE and then passes the serialised caps event to the src pad task for actual handling. At that point, qtmux receives the new caps, checks it against gst_pad_get_current_caps() for the pad (which by now has the same caps as the event), and doesn't produce an error.

It seems to me that fixing that either requires a change in GstAggregator, or storing the current caps separately inside qtmux and not relying on the gst_pad_current_caps() to get the previous caps.