pad: get_allowed_caps() should go through both pads
pad: get_allowed_caps() should go through both pads

The previous implementation was doing a direct call to the peer pad,
which resulted in query probes never being called on the original pad.

Instead of that, get the peer pad caps by using gst_pad_peer_query()
which will call probes in the expected fashion.

centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,krichter722/gstreamer,krichter722/gstreamer,centricular/gstreamer