New floating-point HSL to RGB conversions

PD's old HSL to RGB code uses a weird [-1, 5] range for hue, which makes
all my HSL code wonky.  These new functions use the standard [0, 6]
range.  I'll eventually switch all functions to this version.
Convert Split Toning tool to HSL (instead of HSV)

See http://en.wikipedia.org/wiki/HSL_and_HSV.

When converting from RGB, HSL is more resistant to tonal changes than
HSV.  Why?  HSL treats a luminance value of 1 as pure white, and a
luminance value of 0.5 as "pure color".  HSV treats a luminance value of
1 as "pure color", and a luminance of 1 only represents white if the
corresponding saturation value = 0.

Anyway, because of the way split-toning works, very vivid split tones
via HSV result in a very washed-out image, because white sections get
replaced with pure color.  By using HSL, we minimize that kind of
overflow, and colors are instead maximized in midtone areas (and
absolute white is successfully preserved).

With this fix, I think I'm finally done messing with this tool.  (Famous
last words...?)