Fix a display glitch in the HSLuv color wheel when color is black or white

This MR fixes a display glitch reported in #3175 (closed) and affecting the newly added HSLuv color selector.

After the change, the color wheel no longer disappears when the color is set to pure black (lightness=0.0) or pure white (lightness=1.0).

In addition, minor linter warnings are fixed (struct declared as class, comparison of integers with different signedness).

Fixes #3175 (closed)