Create clipping mask

[peterdesmet]

Create a clipping mask for the flow, based on a 150km radius around each radar. This mask should just hide the flow, not actually stop the drawing in covered areas that are still within the bounding box.

Challenge: how to translate 150km to the pixel space.

[bartaelterman]

Implemented a test by clipping when new particle trails are drawn. I have the impression that this has negative effects on the apps performance.