io: rewrite obj loader to use a specific intermediary model

[mrlem]

In a nutshell:

• have a complete parser to a complete data model, specific to wavefront obj format
• have a mapper from this intermediary representation to siage3d internal representation

.obj → wavefront model → siage3d model

By splitting parsing from feature-mapping, later evolutions due to engine supported features will be simplified, as they won't involve rewriting parts of the parser. This parser could be a separate kotlin lib altogether.

Links of interest

• grammar: https://github.com/antlr/grammars-v4/blob/master/wavefront/WavefrontOBJ.g4
• format description: https://en.wikipedia.org/wiki/Wavefront_.obj_file