Diagonal spring rings tweak

If you didn't know before, those special placement ring lines for diagonal springs only use multiples of 45 for their angles in-game. In other words, they only face any of the 8 basic cardinal directions (N, S, E, W, NE, NW, etc). Considering that springs themselves don't follow the above behaviour, you can probably work out that's a bad thing.

This branch changes that of course, if you couldn't guess from context. Diagonal spring rings can now be placed with any angles like most thing types already could!