Properly parse 'mg' keyword in .obj files

The 'mg' keyword is currently being interpreted as a material library keyword,
when it really refers to the merging group.  Handle this case, in effect ignoring
the keyword as merging groups are currently unsupported.

code/ObjFileParser.h

@@ -102,6 +102,8 @@ private:
 	void getGroupName();
 	/// Gets the group number from file.
 	void getGroupNumber();
+	/// Gets the group number and resolution from file.
+	void getGroupNumberAndResolution();
 	/// Returns the index of the material. Is -1 if not material was found.
 	int getMaterialIndex( const std::string &strMaterialName );
 	/// Parse object name