Allow ICC correction to specify intent

[manthey]

Based on the LittleCMS docs, the intents don't have particularly profound changes and the perceptual intent is clearly the correct default. In practice, perceptual, relative_colorimetric, and saturation are the same for most TCGA samples, while absolute_colorimetric is often a different brightness since the white point differs.

From the LittleCMS tutorial, roughly:
PERCEPTUAL: Hue hopefully maintained (but not required), lightness and saturation sacrificed to maintain the perceived color. White point changed to result in neutral grays. Intended for images.
RELATIVE_COLORIMETRIC: Within and outside gamut; same as Absolute Colorimetric. White point changed to result in neutral grays.
SATURATION: Hue and saturation maintained with lightness sacrificed to maintain saturation. White point changed to result in neutral grays. Intended for business graphics.
ABSOLUTE_COLORIMETRIC: Within the destination device gamut; hue, lightness and saturation are maintained. Outside the gamut; hue and lightness are maintained, saturation is sacrificed. White point for source and destination; unchanged. Intended for spot colors.