app: improve GimpToolLine slider motion clamping/constraint
During constrained motion, round the slider value before clamping it, so that the slider limits are always enforced. Additionally, snap the slider to 1/12-ths of the line length, rather than 1/24-ths. Make sure that sliders can never have negative-zero values, which can result in a -inf base for spiral.
Showing with 4 additions and 2 deletions