Better sun

Makes the sun less stupid.

The sun is now specified by a time of day as well as a rotation (i.e. if you want the sun to rise from somewhere other than the east)

The time of day also influences the temperature/colour of the sun.

I also added an animate option, but it's especially stupid because it rebuilds the BVH every time. Ultimately this leads to a huge amount of overhead so it would be good to fix this eventually.