Fix behavior of Matx 12 and 16 args constructors.
Fix behavior of Matx 12 and 16 args constructors.

The 12 and 16 arguments Matx constructors differs from all others,
leaving values initialized and requiring the argument number to be equal
to the channels number.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv