Fix T49979: HSV/HSL 'Far' interpolation in colorbands when both stops have same hue.

When two stops had the exact same color, 'Far' interpolation would
behave like the three others and generate constant colors, instead of
generating expected 'full turn' in Hue space.

Note that this will break the 'constant' color between two stops with
same value in the 'Far' interpolation mode, but think that change is
OK/expected.

This fix also exposed that 'early out' case when we are after last stop
was not working properly, still triggering actual interpolation
computation in all cases, thus the refactor in `BKE_colorband_evaluate()`
itself. We also now avoid computing factors in constant case, and get
out early in all interpolation cases but the 'spline' ones (where color
ramping can start before first stop, and end after last one).

Work initiated by patch from Charlie Jolly (@charlie) in T49979, thanks.

Reviewers: brecht, sergey

Subscribers: charlie

Differential Revision: https://developer.blender.org/D4556

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