Fix -nomouse
Center only on display 1. No more multimonitor span.
Display error window on crash
Fix mouse warping

In some cases, the warp back to center was being detected as a mouse motion, causing all sorts of silliness with the mouse. The workaround is by only using the first motion event and ignoring every event after that, until the next call to I_GetEvent.
Use scancodes instead of keycodes for locale independence.

ONLY US KEYBOARDS SUPPORTED! THAT'S HOW WE DO IT IN AMURRICA
Fix MOUSE4/MOUSE5

MOUSE6-8 will not be supported on SDL2 as there is no interface for them.
Virtual resolutions in OpenGL

Also made fades use core functions if they are available.