SDL2: Relative mouse mode

This replaces the old warp-mouse-to-center continuously hack with SDL2's relative mouse mode. Probably fixes the issue reported here: https://mb.srb2.org/showthread.php?t=42848 This was originally started by @Alam, but then I took over to fix problems with the mouse sensitivity and do further cleanup of my own. I think it's pretty ready go in now, give or take some more cleanup.

Another bug I fixed in the process of improving this branch was this possibly obscure one I've known for a while now, where moving the mouse into the window can suddenly jerk the camera unexpectedly.

I've only tested this with Windows that said, is anyone able to check how well this works for Linux/Mac builds?