280: LOD Additive Mode validation for no double draw

[tngreene]

Suppose an OBJ with (0, 200), and (0, 400) LOD buckets. This is Additive Mode. Next suppose CubeA is in Bucket 1 and 2.

For no good reason, CubeA will be drawn twice! X-Plane can't optimize this out and it is potentially a big performance suck that would be difficult to debug. After all, it is simply a one checkbox mistake.

A warning for this case would be good.