Implicit conversions in SRGBColour should not exist

[frenzibyte]

As part of making sense of sRGB colours. It should be explicitly shown whether SRGBColour is going to give a linear or sRGB colour, by getting rid of the implicit conversions in SRGBColour and potentially replacing them in public API with just pure Colour4s.

This should make it clear in what space the colours passed to the shader are, and hopefully reduce confusion on sRGB colours in general.