INFINITY/NEG_INFINITY consts for vectors

[LPGhatguy]

Inline with Vec3::ZERO, Vec3::NAN, etc, it would be helpful to also have positive and negative infinity as constants. These values come up a lot when refining bounds on objects, like for calculating bounding boxes.

[bitshifter]

Yep, I think that makes sense.

[bitshifter]

Merged in #403