replace (unspecified) exm json with binary chunk

[shakesoda]

parsing json is very slow, a hassle to do when using a language that doesn't provide it (i.e. C, C++), and inflates file sizes.

need to think about an efficient and simple structure for the scene metadata, and switch it from using a json comment to iqm extensions. no header changes will be required, so this will only impact compatibility if you use the existing json data.