function fromXRotation(out, rad) {
    var s = Math.sin(rad);
    var c = Math.cos(rad); // Perform axis-specific matrix multiplication

    out[0] = 1;
    out[1] = 0;
    out[2] = 0;
    out[3] = 0;
    out[4] = 0;
    out[5] = c;
    out[6] = s;
    out[7] = 0;
    out[8] = 0;
    out[9] = -s;
    out[10] = c;
    out[11] = 0;
    out[12] = 0;
    out[13] = 0;
    out[14] = 0;
    out[15] = 1;
    return out;
  }
Creates a matrix from the given angle around the X axis
This is equivalent to (but much faster than):

    mat4.identity(dest);
    mat4.rotateX(dest, dest, rad);

@param {mat4} out mat4 receiving operation result
@param {Number} rad the angle to rotate the matrix by
@returns {mat4} out