[visibility] Add EPS(ILON) for float comparisons
[visibility] Take into account known/unknown state of the points
[test] update visibility unit test

Take into accourt the unknown/known state for visibility.
[nav_graph] cleanups
[weight] s/HUGE_VALF/numeric_limits::infinity/g
[gdal] scale must be positive

Especially when computing weight for A* (!)
[python] fix gladysplay points scaling