Currently, the film-like tone map curve evaluates the minimum and
maximum channels on their own curve maps. This is undesirable, because
the film-like curve can only correct for small hue shifts, and
evaluating the channels on different curves can results in big shifts
that can't be corrected and may produce artefacts.

This patch evaluates all channels on the same master curve and hides the
rest of the curves from the UI.

Additionally, though perhaps not related to the task, the film-like
implementation ignores the white balanced values but uses them to
generate the appropriate permutation. Which also contributes to the bad
output if the white and black levels are not the default.

This patch additionally pass the white balanced values to the film-like
evaluation function instead of the original values.