pad: add "offset" property to go with gst_pad_set_offset()
pad: add "offset" property to go with gst_pad_set_offset()

So we can set the offset via gst-launch.

krichter722/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,mparis/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,mparis/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,shelsonjava/gstreamer