function str(a) {
    return 'mat4(' + a[0] + ', ' + a[1] + ', ' + a[2] + ', ' + a[3] + ', ' +
                    a[4] + ', ' + a[5] + ', ' + a[6] + ', ' + a[7] + ', ' +
                    a[8] + ', ' + a[9] + ', ' + a[10] + ', ' + a[11] + ', ' + 
                    a[12] + ', ' + a[13] + ', ' + a[14] + ', ' + a[15] + ')';
}
Returns a string representation of a mat4

@param {mat4} mat matrix to represent as a string
@returns {String} string representation of the matrix