Custom clear color

[slipher]

The other day @illwieckz suggested a configurable color for r_clear, as a debugging tool. This is a good idea: when I tried this the other day, I uncovered many hidden voids in our stock maps.

But also, what about a custom solid sky color for maps? I think it would be cool to let maps specify a color in the worldspawn that would be used for clearing when r_drawSky is off.

So I propose to add both mechanisms. The cvar can be a cheat cvar that takes precedence over the map's color when set. Also when r_drawSky and r_clear are both on, the map color should not be respected, so that we don't reveal more voids than necessary :)

For color cvars that don't need to be configurable via a menu, I like to use a string parsed by the color code parser (^1, ^xf0f etc.).