Add sRGB colour mapping

[Andrew Baldwin]

The current 3 colour mappings don't give a "normal" view because they are either linear, log, tone mapped, but not with normal monitor gamma corrections.

There should be an additional option for a normal sRGB gamma curve which would give the most conventional looking colour.

[Andrew Baldwin]

Will also do Rec.709 gamma curve

[Andrew Baldwin]

Introduced in f7d3d63b1ecf51b002b866f9dd4c69c1bad05f98