Frustum::from_matrix4 is wrong

[flodiebold]

For example,

let persp = cgmath::PerspectiveFov {
    fovy: cgmath::Deg { s: 90.0f32 },
    aspect: 1.0,
    near: 0.1,
    far: 1000.0
};
let frustum = Frustum::from_matrix4(persp.to_matrix4());

println!("{}", frustum.far);

println!("{}", persp.to_matrix4().mul_v(&Vector4::new(0.0, 0.0, 1.0, 1.0)));
println!("{}", persp.to_matrix4().mul_v(&Vector4::new(0.0, 0.0, -1.0, 1.0)));
println!("{}", persp.to_matrix4().mul_v(&Vector4::new(0.0, 0.0, -999.0, 1.0)));
println!("{}", persp.to_matrix4().mul_v(&Vector4::new(0.0, 0.0, -1001.0, 1.0)));

outputs 0x + 0y + 0.000999z - 0.999999 = 0 for the far plane, i.e. z=1000, when in fact the plane should be z=-1000.

I think this is because Plane::from_vector4 uses the vector's fourth component as D, but the way from_matrix4 uses it, it should be -D. For example, the left clipping plane should have the equation v . (row(3) + row(0)) = 0 (taking the rows from the projection matrix), but since planes are represented with equations of the form A_x + B_y + C*z - D = 0, the D is negated.

I'm not sure whether Plane::from_vector4 is doing what's intended, but either it needs to negate D, or Frustum::from_matrix4 needs to be fixed somehow.