Support for (optional) shadow meshes

[past-due]

Reference: https://github.com/Warzone2100/warzone2100/pull/1630

It may be possible to generate a simplified shadow mesh on export, with various adjustable parameters. Alternatively, offering the ability to explicitly specify a mesh as a shadow mesh is something the Blender plugin is planning to support.

At a minimum, preserving the shadow mesh data from .PIE files is important (from import -> re-export).