Better handle near and far camera values depending on camera position

[jailln]

The near and far plane are fixed in the GlobeView and the PlanarView.

1. Make them configurable
2. Update the far distance dynamically depending on the camera position. The far plane is indeed quite far (10 times the earth diameter for the GlobeView) to allow zooming out pretty far from the globe or the plane. This value is not adapted when zooming in and tilting the camera: it generates unuseful tiles subdivision and external layers querying and manipulation, which has an impact on performance. Ideally, this should be configurable in some way too.

[gchoqueux]

Yes, it's good idea, i started in this branch https://github.com/gchoqueux/itowns/tree/nearFar_2.0
You could use the global bounding box to compute the near and the far.