mint-arena: Fix vid_restart while in-game not setting up viewport

Causes text not to render at info screen and crash when opening console.
mint-arena: Make sure console field isn't bigger than buffer
Fix an invalid null deref check in the slider code.
mint-arena: Allow disabling skybox ground surfaces

RTCW SP allows disabling the non-sky surfaces. In RTCW SP, disabling
the sky box actually only disables the ground in the skybox.

Put this behavior at cg_skybox 2 instead.
Merge remote-tracking branch 'mint-arena/master'