distance and colour scales

[chrfi]

it would be good to add a scale for the distance on screen and the colour by height to lag. prefereably these would be in the main viewing window in one corner floating above the opengl drawn flight line.

they need to be togglable and easy to read against the black background and most of the colours the flightlines can be.

[harg]

I have added a distance scale and a colour scale to the overview and an absolute height scale to the profile view. These are all toggle-able and the writing and, for distance and height, the scales are in white against a black background.