Implemented non-linear colour and pigment map interpolation.

The syntax is changed as follows:

    COLOR_MAP:
      color_map { COLOR_MAP_BODY } | colour_map { COLOR_MAP_BODY }
    COLOR_MAP_BODY:
      COLOR_MAP_IDENTIFIER | [blend_mode INT] [blend_gamma FLOAT] COLOR_MAP_ENTRY...

    PIGMENT_MAP:
      pigment_map { PIGMENT_MAP_BODY }
    PIGMENT_MAP_BODY:
      PIGMENT_MAP_IDENTIFIER | [blend_mode INT] [blend_gamma FLOAT] PIGMENT_MAP_ENTRY...

The possible values for blend_mode are as follows:

    0 (default): Colour interpolation is performed working gamma space as defined by assumed_gamma.
    1: Colour interpolation is performed in linear colour space.
    2: Colour interpolation is performed in the gamma space defined by blend_gamma (defaulting to 2.5).
    3: Chromaticity interpolation is performed in linear space, while brightness interpolation is performed in the gamma space defined by blend_gamma.