Fix #109: Seg faults caused by wxradar with ARC display mode (aerostar700)

Fix #109: Seg faults caused by wxradar with ARC display mode (aerostar700)
Fixes an ugly issue with OSG overwriting memory/stack. wxradar triggered
the issue in display mode "ARC" - since it configured a display mask (12
vertices) but all vertices were immediately removed again ("_vertices->clear()").
Issue is fixed, mask now working - however the mask wasn't even properly
aligned for the display (probably never worked since OSG port). The display
mask is only really required for weather display - so disabling for now.