Add mat4.trace.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add mat4.fromArray/diag.  (Non asm.js).

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

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