Using an approximated pow function improves the conversion speed of LAB images by more than 50% without an appreciable visual difference.
LAB images support: - Added LAB 8/16-bits support by converting them to sRGB - The conversion are done using literature formulas (LAB -> XYZ -> sRGB): [sRGB on Wiki](https://en.wikipedia.org/wiki/SRGB), [LAB on wiki](https://en.wikipedia.org/wiki/CIELAB_color_space) - Removed unused code
