Cleanup the sphere intersection function

[Carlbaum]

if (t0 < 0) { t0 = t1; } if (t0 < 0) { return nullptr; }

if t0 is actually less than 0 we check it twice.. should probably be something like: if (t0 < 0) { if (t1 < 0) { return nullptr; } t0 = t1; }

[novalain]

Yeeee this is a mess. Compare and maybe go back to the old sphere intersection method