Blender Git Loki

Eeshlo AO patch, revised

- Ambient Occlusion is a more sophisticated ambient trick, which takes
nearby faces into account by firing a hemisphere of shadow-rays
around. AKA 'dirt shader'.
- Eeshlo made it a Lamp type, which doesn't fit well. I've moved the
settings to the World menu, and let the Material->ambient value control
the amount it contributes
- currently, the AO value is added/subtracted/mixed with the 'diffuse'
factor while shading, before it is multiplied with Material color

Buttons are in new Panel 'Amb Occ" in F8 menu. Note:

- "Dist:" by shortening the length of rays you get subtler effects and it
renders faster too
- "DistF:" the attennuation factor gives control over how the 'shadow'
spreads out.

Further it's just raytracing, so tends to be slooooow.... :)
Here same tricks as for other raytraced scenes apply, especially try to
keep the environment as small as possible (exclude faces from Octree by
giving them no Material Traceable).

I still have to think over a couple of aspects, will await feedback on it:
- AO color? Now it just adds 'white'
- other sampling patterns? I tried dithering, which was so-so
- method of controlling final 'samples' in F10? Might be useful for other
oversampling too (area light) to have it reacting to a percentage or so..