Fix quicksand

[WilliamFr0g]

Objects currently sink at different speeds in quicksand, sometimes so badly that they will sink upwards. This should fix that

[qixils]

This replaces the gravity local variable, which alesan added in 957d534 supposedly to "Fix quicksand speed varying with FPS and gravity", with b.gravity, leaving the gravity local variable unused. An issue with quicksand clearly still remains (prior to this PR) but it feels like the solution should be to fix the gravity local or to remove it entirely if proven not to be necessary. Additionally, does this reintroduce the FPS dependency alesan mentioned and attempted to fix in the prior commit?

[WilliamFr0g]

You could probably reimplement the gravity local, it may work better with low gravity that way, but I hadn't considered that at the time. In that case, it should still account for ignoregravity, which it didn't before. The FPS dependency has never been fixed until I did here; Alesan's last commit claimed to fix it but did not.