Ambient cleanup

Remove ambient color property from lights, scene ambient is enough. Fixes overdark building models from #1923.

Removed building fade-in lighting when flying close to them, the code was... complicated and I think it looks fine without it. If it's an important feature important to have, we can try something simpler.

But, first thing you'll notice is I removed ambient lighting from intro so it's not so flat, I think it's nicer :-)