Document that Matrix4::look_at_rh and Matrix4::look_at_lh can output NaN values

[est31]

If your looking vector is too close to being parallel to the axis you are passing to Matrix4::look_at_rh, the return value can contain NaN values. This apparently seems to be expected behaviour. However it's a bit surprising if you don't know about this. It should be documented IMO.

[est31]

It is documented here: https://www.khronos.org/registry/OpenGL-Refpages/gl2.1/xhtml/gluLookAt.xml

The UP vector must not be parallel to the line of sight from the eye point to the reference point.

[est31]

Also maybe some way of doing this computation in a checked way would be nice, so that you can adjust the vectors. Right now there is a lot of guesswork involved :).

[sebcrozet]

I agree this should be documented.
We could add methods like try_look_at_rh that takes an epsilon and return an Option if some norm end up being close to zero.