Merge branch 'gasjets_and_slopes' into 'next'

Gas jets, fan objects, springs and slopes

Whoop whoop whoop minor bugs that only get noticed due to weird experiments whoop whoop whoop

* For gas jets only: the object's standingslope is ALWAYS null. No drifting down the slope for you.
* For gas jets and fan objects: Now checks whether the player's top is below the bottom of the launcher, instead of its bottom. zdist calculation not affected, since it's signed and okay with being negative by about the height of the player.
* For all 3: the player's standingslope is NULL'd so the player isn't launched off at a wacky angle if they're standing on a slope then walk into the thing.

See merge request !91