Fix bug when ContentReader is not a MemoryStream & data needs processing

Missed this case before. When the ContentReader is _not_ backed by a MemoryStream, we're not guaranteed that levelData is null — it's entirely possible to, for example, load a DXT compressed texture from a FileStream on a system without the right GL extension, in which case we would've already read in levelData and decompressed it.

The documentation is really nice, btw! :)

— David