Add mat4.multiplyScalar.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add mat4.rotateX.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add mat4.rotateY,rotateZ and proper tests.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>