Clean up steam/gas jet behavior.

This merge request brings the behavior of steam/gas jets more in line with other springs, by utilizing the MF_SPRING flag and actually using the object's height for collision. This removes the need for the GasJet part of P_DoFanAndGasJet, so that function has been cleaned up and renamed to P_DoFan.

It also slightly increases the height of the gas jet's hitbox, makes it stay around longer to better match the new sprites. If that is somehow undesired/unfit for a patch, those changes can be undone. Amy can also gain extra height from gas jets now, but I personally don't see that as an issue.