Automatic exposure

[GSterbrant]

Description

The exposure with the physical camera units #4585 and physical light units #4618 means we can have really big ranges of light intensities in the scene. Without automatic exposure, it could mean a badly fit set of exposure settings would cause the entire screen to go black or white due to under or over exposure respectively.

To ameliorate this, we should implement an automatic exposure feature where we generate a histogram of the light intensities, and automatically adjust the camera exposure settings accordingly.