Spring fixes

Remember how a while back I fixed things like being stuck under a spring after jumping from below? Or how you could touch multiple springs at the same time and subsequently cause a loud racket? I accidentally broke something else in the process it turns out! (this sounds familiar...)

Namely, diagonal springs can't send you to their centers anymore, because of several changes I made to player-spring collision towards the other issues. This branch fixes that particular issue, while making sure none of the old bugs were broken again. Took a lot of testing to make sure of this.

As a bonus, the P_DoSpring function returns a boolean now, which can be used by Lua for whatever: true = you touched spring; false = you didn't