Duke3D: Color correction menu allows negative brightness while expected lower bound is 0.0

Originally reported here: https://forums.duke4.net/topic/11290-color-correction-settings-not-keeping-when-starting-next-session/

There's an inconsistency in the brightness setting of the color correction menu of Duke 3D. Namely, the Duke 3D menu allows the user to move the brightness slider below 0.0 down to a lower bound of -0.8.

However, build/src/baselayer.cpp, build/src/config.cpp and duke3d/src/osdcmds.cpp all expect vid_brightness to be constrained to a range of [0, 10], and will clamp this value if it is out of bounds.

One way in which this manifests itself ingame is that the game will not save the player's chosen brightness in the CFG on exit if it is less than 0.