#11 Multi-monitor support

[Eric Wasylishen]

This is all SDL2 only, I believe:
- Alt+Enter (VID_Toggle) should keep the window on the same monitor (so you can drag the window from one monitor to another, and it goes fullscreen on which ever monitor it is currently on). We need to use SDL_SetWindowFullscreen, addressing the comments in VID_Toggle
- We could have a menu option for choosing the monitor.