Matrix.multiply
Description:
Multiplies the two supplied matrices together and stores the resulting matrix inside this.
Note: 
      
        
    
    It is possible to multiply the a matrix in-place, so this can be equal to a but
          can't be equal to b.
Parameters:
| this | 
           The address of a 4x4 matrix to store the result in  | 
      
| a | 
           A 4x4 transformation matrix  | 
      
| b | 
           A 4x4 transformation matrix  |