Add IMeshTextureLoader interface and replace texture-loading algorithms in most meshloaders.
Previously each meshloader had implemented texture loading independendly. This had caused a lot of redundant codes and behaviour 
between different meshloaders was often different. So we have now one common interface which should be used by all meshloaders.
This also allows deprecating TEXTURE_PATH attributes as those paths can now be set by that interface and for all meshloaders using it.
I have replaced the codes for the 3DS, B3D, CSM, Collada, DMF, LMTS, LWO, MY3D, OBJ, OCT, Ogre, SMF and X meshloader.
Not changed (just using old code) are: MS3D (I forgot that one), CIrrMeshFileLoader (haven't figured out how it works yet).
STL and PLY both don't load textures and need no changes.
MDL, MD3, MD2 and BSP all work a little different (the textureloading there isn't implemented inside the meshloaders). Those could maybe still be adapted.
Stuff still needs a lot more test (unfortunately I don't have test-models for most formats...), but wanted to check it in already to get some feedback.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4703 dfc29bdd-3216-0410-991c-e03cc46cb475