Returns a colour or pixmap lightness inverted.
cis a colour
pmis a pixmap
c is a colour, then
InvertLightness(c) returns the lightness-inverted colour of
c. This is obtained by replacing the lightness with its complement, that is,
InvertValue(hsl(h, s, l)) = hsl(h, s, 1 − l).
pm is a pixmap, then
pm with every pixel lightness inverted as described above.