[is0] Mesh exporter from SDF

[JulesFouchy]

Add the command is0Command_ExportMesh{ ModuleId id; };

usefull links:

• https://0fps.net/2012/07/12/smooth-voxel-terrain-part-2/

• Add a button in the export menu : "3D Model" (and if the current main node is not a 3D SDF, show an error message (but first try to go up the graph and find a 3D SDF somewhere))

• Show the bounding box of the part of the scene that will be meshed