Parse the OBJ file format for the name of the MTL file.

[capnmidnight]

Currently, the ModelFactory class assumes an MTL file of the same base filename as the OBJ file, i.e. it just replaces the extension. But that may not always be the case.

The OBJ file specifies which MTL file it uses, so we should load the OBJ first, find out which MTL file it wants, and load that instead.