Allow making maps designed for tr.overbrightBits greater than 1

[slipher]

Map overbright bits (r_overbrightDefaultExponent) defaults to 2, while r_overbrightBits defaults to 1. This configuration was always used for Quake 3 and Tremulous, but really it is stupid because you are throwing away one bit of precision: the [0, 1] values from the lightmap are multiplied by 4, but then they are clamped to be no greater than 2. We should have a worldspawn key for setting tr.overbrightBits (in addition to the one setting tr.mapOverbrightBits) so if someone wants to make a map with really bright areas that should range [0, 4] or even [0, 8], they can use it.

I think we would still have this problem even with sRGB, since AFAIK the lightmap then still ranges from 0 to 1; the increments are just different.

Note that making a map with map overbright bits 0 would already work fine since tr.overbrightBits is constrained to be no greater than the map overbright bits.