Merge: Extract sdl, mnit_display and mnit_input from mnit

Merge: Extract sdl, mnit_display and mnit_input from mnit

This is the beginning of the division of mnitt into a portable application framework (paninit) and the game framework (gammit). For now, allows to write an SDL application without all of the portable part of mnit.

Also solves problems as noted in #236.

#193 will be fixed after the numerics are merged.

Pull-Request: #384
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>