matrix4x4to3x3

Matrix4f m1 = matrixf(
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, 1, 0,
    0, 0, 0, 1
);
Matrix3f m2 = matrix4x4to3x3(m1);
assert(m2 == matrixf(
    1, 0, 0,
    0, 1, 0,
    0, 0, 1)
);