patch: [#30986] 3DS importer/exporter material improvements and bug fixes.

from Alexander Gessler (aramis_acg)

This patch extends the 3DS importer and exporter to have better material support and unifies their respective feature sets.

This includes:
- UV scale,offset
- tiling/wrap modes
- improved conversion of mapping targets
- better warnings

Additionally, the exporter now handles specular, opacity and normal maps correctly (the importer did this already). (I think the way how textures were written previously was non 3DS-compliant).

I tested this against assimp's 3DS test suite [1] as well as my personal 3DS test sets. Exported 3DS files were successfully read by FBXConv and assimp.

[1] http://assimp.svn.sourceforge.net/viewvc/assimp/trunk/test/