Caves are too dark

[careeoki]

Pitch dark caves are cool and scary, but would suck for gameplay. You can't see the ground you are standing on, let alone the cave around you: any cool generation, any ores or points of interest.

[IntegratedQuantum]

I would like to keep the basic cave biome in 100% darkness. I don't want the base caves to be lit, to avoid the boring gameplay of turning your brightness up instead of placing torches.

So I think we should give each biome a base light level parameter (and while we are at it, maybe also a base fog color/density), that get set in a shader uniform. A smooth transition should be used when changing biomes.

[ikabod-kee]

This issue might not be relevant with #728

[careeoki]

This didn't end up being as much of a problem as I thought it would. And with lava caves, crystal caves, and glimmergill caves, caves can be pretty bright.