allows using Mat and vector<Vec3d> as argument for rvecs, tvecs that are
allocated at once.

opencv_dnn: replaced static LayerFactory::impl member with a static function that constructs the object on the first use (fix for bug #383)

This way, we are certain not to depend on the order of static
variable initialization (LayerFactory::impl in dnn.cpp, vs. init
in init.cpp), which is causing bug #383.

also avoids manual memcpy: the predefined dictionary data is not
duplicated any more.