[CR] SDL fullscreen support

Added some simple fullscreen toggle functionality for SDL. It recreates the window with SDL_FULLSCREEN flag if this flag wasn't previously set, and recreates the window without SDL_FULLSCREEN flag if it was previously set. Toggle-key is F11, which is hardcoded.

SDL_FULLSCREEN stretches the screen to fit the monitor.

Tested on Windows 8.1 and working.

A minor bug is present, in order for the screen to redraw after toggled the state some form of input is needed. Otherwise the screen will remain black after toggling.