Implement Camera support with OpenGL

[wmanley]

TODO:

• [x] Merge #311
• [x] Merge #313
• [x] Merge #342
• [x] Rebase
• [ ] Update stbt camera to use the OpenGL transformations.
• [ ] Test on the big rig with a real TV
• [ ] Reinstate Vignetting correction
• [ ] Investigate more correct colour management? (See ArgyllCMS)
• [x] Remove gst_hacks.gst_pad_stream_lock
• [ ] Move run_on_stream_thread from gst_hacks.py to gst_utils.py

[drothlis]

This code now corresponds to the demo at the 2015 GStreamer conference, rebased onto master. The OpenGL pipelines are driven from an iPython notebook -- stbt camera calibrate still needs to be updated to use the OpenGL transform.

[wmanley]

Closing: stbt camera support removed in #589