effects/colorpicker: Fix off-by-one error during coordinates conversion

Due to this bug color picker used to not only select one pixel higher
than it should (which is quite negligible), but also returned garbage
value for the topmost row of pixels (when you push the cursor against
the top edge of the screen).

FIXED-IN: 5.24.7, 5.25.5, 5.26
(cherry picked from commit f892c951)