FIX: prevent switching to windowed mode if windowing do not support it

Problem is that switching to windowed only displays a scaled display (720x480) on the bot-left on the screen and users can inadvertently switch to window mode by hitting "\" on a keyboard. 

@jmarshallnz There must be a better way, but I cannot find it.
Apparently, there is no provision to deny RES_WINDOW anywhere...