>  The only thing I know for sure about gamma correction is that whenever
> it makes an appearance, it only causes problems and nothing else.

So let's simplify things for POV4:

Facts:
* A raytracer needs to work internally in linear colour space
* A monitor is a non-linear device (usually requiring a gamma correction of 
1.8-2.2)

Assumptions:
* Most outputs from a raytracer will be in monitor colour space, not linear 
space
  --> raw output from the raytracer needs to be converted from linear space
* Most inputs to a raytracer will be in monitor colour space
  --> User input needs to be converted to linear space

Default solution:
* One single system-defined Gamma value (default 2.2 for Windows, editable 
by user)
* Gamma applied to all output from raytracer engine
* Inverse-gamma applied to all input to raytracer engine
* For #version <4.0 the old behaviour is set (ie no conversion for input 
colours)

Options:
* User can change Gamma value for their system (in .ini file?)
* User can use SDL keywords to set if input values in that file are 
specifically in linear space

eg: color rgb <1.0,0.5,0.25> will be the same colour as RGB 255,128,64 when 
viewed on your monitor, no matter what gamma setting you have.

linear_color rgb <1.0,0,0> will be twice as bright as linear_rgb <0.5,0,0>. 
Can be used when you want a linear mapping for physical reasons (eg one box 
twice as bright as the other).

There could also be a linear_image_map keyword to work with texture files in 
the same way.

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