Tunnel camera preview output port to null sink input port in Raspberry Pi

According to [1] using preview port contributes to the data used by
algorithms controlling the camera. So lets open and use the preview port
but discard the images by sending these to the null sink component.

[1] http://www.raspberrypi.org/wp-content/uploads/2013/07/RaspiCam-Documentation.pdf
Probe hardware for supported output formats and encoder support for RPi

Camera output port is now queried for supported output formats. Also on
Raspberry Pi, it's possible to use the encoder component with camera
output. For instance, you could record a 10 second clip of H.264 encoded
video encapsulated in a Matroska file like this:

    gst-launch-1.0 -e omxcamerasrc num-buffers=250 ! \
        video/x-h264,width=480,height=270,framerate=25/1 ! \
        h264parse ! matroskamux ! filesink location=/tmp/test.mkv