SDL2 complete

The SDL2 interface is now feature complete and ready to be merged into mainline.

Here are some things that still need to be fixed, but can be left on the master for now, until SRB2 2.1.9 is ready to launch:

* Fix ping message struct packing format on Windows/32-bit (inconsistent with linux64 for some reason)
* Figure out what's causing strange behavior with XInput controllers on Windows