Support resizing windows + improve fullscreen for SDL2

By implementing GLimp_SetScreenParms() and enhancing the
event handling
Support resizing windows for SDL1.2

By implementing GLimp_SetScreenParms() for SDL1.2
and enhancing the event handling
support ctrl-g for (un)grabbing mouse

Many Linux Games support that
Pause when window loses focus, introduce com_pause

If the window loses focus com_pause is set to 1, when it regains focus
it's set to 0.
The behaviour on Win32 stayed the same (the implementation is less
hacky though) and Linux now matchces that.