+ Fixed bug in SGSHAPE::Prepare() : bad assignment of color pointer

+ Reworked s3d_plugin_demo2.cpp to use per-vertex-per-face normals
+ Reworked s3d_plugin_idf.cpp to use per-vertex-per-face normals

include/plugins/3dapi/ifsg_api.h

@@ -42,6 +42,12 @@ struct S3D_POINT;
 
 namespace S3D
 {
+    /**
+     * Function CalcTriNorm
+     * returns the normal vector of a triangle described by vertices p1, p2, p3
+     */
+    SGLIB_API SGVECTOR CalcTriNorm( const SGPOINT& p1, const SGPOINT& p2, const SGPOINT& p3 );
+
     /**
      * Function WriteCache
      * writes the SGNODE tree to a binary cache file