FileFormatTruelight: Added write support for truelight luts

Hi,

Just added write support for Truelight luts (something we were wanting to use with ociobakelut). I've just put in a unity shaper lut for now, as this is what Nuke does (brokenly I might add). If there is interest I can probably put in an auto detected shaper lut too, but that would be a bit of work and is not on my immediate plans.

Cheers,
Mark