Add tools for flattening 3dtiles

[qwork2010]

In practical work, it involves flattening the tilted photography model in 3dtiles format. Please add corresponding tools so that it can be flattened according to polygons.

增加压平3dtiles的工具
实际工作中涉及对3dtiles格式倾斜摄影模型的压平处理，请增加相应的工具，以便可以根据多边形压平。

[Ryan Veenstra]

You can use cartographic polygons to remove sections of terrain / photogrammetry / 3D tiles - see this tutoral here

Please let us know if there is a specific reason that clipping terrain via polygon does not work for your particular use case.