Rework the LOD ranges.
1. The scenery ranges are now deltas (avoids overlapping values)
2. The AI/MP pixel mode now has a default radius that is 20 for Aircraft, 200 for ships, 350 for carriers. This is a simple constant in a virtual function.
3. Added the ability to set the AI/MP ranges equal which means use the low detail model.
4. Changed high detail only to be indicated by a -ve number in maxRangeDetail
5. Re-ordered the range list to go from lowest detail at [0] to highest detail at the end. This is because OSG always loads the models starting from zero on the assumption that the detail increases with the index.
This fixes the pixel mode, which previously would use the radius of the parent which would be confusingly large, and unrelated to the actual size of the model. With the simple defaults that we have the pixel values set in the ranges won't exactly match the rendered size of the model on screen, but it will be a lot closer and more importantly meaningful.
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