Merge branch 'polyobject-checksight-fix' into 'master'

Polyobject checksight fix

This branch fixes how polyobject lines apparently stop objects from "seeing" other objects behind them; the relevant code now checks the polyobject's top and bottom too! This is relevant for enemies trying to get you, Homing attack, Fang's gun, etc etc.

Can't say if it's perfect or not, but it's a definitely an improvement over what happened before, anyhow.

See merge request !86