Show FENCE_ENABLE visually with dashed lines

[Ryanf55]

If the fence is disabled, I want to see dotted lines. In MAVProxy, there is no distinction in the map. The console has a FENCE icon that changes from green to gray, but it's hard to notice that the vehicle isn't going to obey fences if they are disabled.

Example from Cesium:

• https://sandcastle.cesium.com/?src=Polyline%20Dash.html&label=Geometries