qtdemux: Provide a 2 frames lead-in for audio decoders

AAC and various other audio codecs need a couple frames of lead-in to decode it properly. The parser elements like aacparse take care of it via gst_base_parse_set_frame_rate, but when inside a container, the demuxer is doing the seek segment handling and never gives lead-in data downstream. Handle this similar to going back to a keyframe with video, in the same place. Without a lead-in, the start of the segment is silence, when it shouldn't, which becomes especially evident in NLE use cases.