HIL: reversals can now be done with plane RC params
cameraview module to draw a polygon showing fov.

Assumes the terrain is all flat in the field of view.

Stabilisation-corrected code is untested and likely to be quite wrong.

Stole a couple of functions from mavproxy.py, which should probably be
moved somewhere else for the sake of cleanliness...
mavflightview: support EKF position viewing