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