TODO: Reduce banding in darkened areas

[keijiro]

Increasing Strength more than 1.0 introduces banding in darkened areas. This is noticeable in the Statue test scene.

This banding can be removed by using half-float textures for storing AO, but this is not a preferable solution because it requires more memory and bandwidth.

There are two possible solutions:

• Use gamma encoding/decoding to increase the precision of darkened areas.
• Apply dithering to reduce banding.

The former approach might be preferable because of simplicity.