Changed, Visual mode: adjusted fog density calculation to better match with GZDoom.

Fixed, Visual mode: alpha-based texture picking didn't work correctly on HiRes images.
Fixed, Visual mode: fog density was calculated incorrectly for things lit by dynamic lights.
Fixed, Edit Sector window, UDMF: "Fade" and "Light" color pickers initial values were incorrect when displaying mixed values.
Fixed, Edit Thing window, UDMF: "Color" color picker initial value was incorrect when displaying mixed values.