Fix and simplify computation of painter monochrome mode

Fix and simplify computation of painter monochrome mode

The old code in GuiPainter::filterColor did not work. Tricks with
colors should take place in HSV space, not RGB IMO.

Replace the code with a simpler one which maps the grayscale value
of the original color on the blend color. It works nin the case where
original color is red, but might not work as well when blend color is
not black. Time will tell.

Fixes bug #11904.