Model import culling for detailed models

[Florian-Katerndahl]

Working with highly-detailed models, the CityEngine may become unresponsive and/or tasks take unproportionally long.
One idea could be to import models on a snapshot-basis by first positioning the camera and only then importing trees that are either

1. either in a circular area on the groundwhich is drawn from the camera, buffered by X units or
2. handeled as above but also checked against occlusion by other models such as buildings; (trees could be represented as cylinders with a unrealistic height)