vengi voxedit exporting and re-importing gltf

the gltf format got animation support for import and exporting

The windmill model is voxelized, then animated in the vengi voxedit editor, saved as gltf with animations. The produced gltf file can also be converted back into voxels - including animations with the vengi voxconvert command line tool, or the vengi voxedit voxel editor.