Cleanup pad construction using new baseaggregator API
Port to new Aggregator buffer management API
basemixer: The aggregator subclass has to get_buffer

Conflicts:
	gst/videomixer/basemixer/gstbasemixer.c
Make flush_start_flush_stop test unracy and adapt it to now behaviour
mixer.srcpas runs its own thread, fix the test
Fix some locking mistakes
basemixer: check eos before getting buffer