Extrusion node texture mapping

Textures are now correctly mapped onto Extrusion parts (trunk and begin/end caps).

Tested with NIST conformance suite and with another textured Extrusion nodes created by myself.

Not sure about creating crossSection member on SFVec3f vector object (that serves for storing related cross section used later on), but I think it should not do any harm:

pos.crossSection = crossSection[j];