CUBZH: add support for b64 files transforms and references

[mgerhardy]

b64 files are base64 encoded maps for cubzh - they are basically a scene description for 3zh files

the transforms are not applied correctly.

if a 3zh files is already loaded, we should only create a reference, and not load it another time.

See https://github.com/vengi-voxel/vengi/blob/master/src/modules/voxelformat/private/cubzh/CubzhB64Format.cpp