Function mul

mul(a, b, dst?): Mat3
Multiplies two 3-by-3 matrices with a on the left and b on the right (same as multiply)
Parameters
- a: Mat3
  The matrix on the left.
- b: Mat3
  The matrix on the right.
- Optional dst: Mat3
  matrix to hold result. If not passed a new one is created.
Returns Mat3
The matrix product of a and b.