Rework the math around dithering

[LittleEndu]

The math around dithering is crazy complicated and doesn't event make sense. The slider in the settings should set a "show every Nth pixel" and the priority mask should add a percentage to that. There should be a separate slider for the automated dither part of animated frames.

The final formula for finding the dither amount could then be floor(N + N * (255-L)/100) where N is the selected dither amount and L is the value of the pixel in the priority map.

[LittleEndu]

I'm honestly considering dropping priority mask support. Would simplify so many parts of the overlay system