function fromXRotation(out, rad) {
    var s = Math.sin(rad),
        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