rpicamsrc: add test-color-balance example
rpicamsrc: add test-color-balance example

This small test will display a live video preview of the rpicam with
the balance controls being updated once a second. The controls to
update can be disabled in the source by setting the CONTROL_* macros
values to 0.

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good