WIP: Smudge Lightness options

This changes the behavior of Lightness Strength in the new Smudge Engine to match its behavior in the Pixel engine when Color Rate is full, and to give the user three different options for how to smudge at low Color Rates (smudge color only, or smudge height with color) and handle blending of brush lightness values with canvas lightness values (brush takes priority always, or lightness strength value dictates priority). This also changes the name of the Lightness Strength option to Paint Thickness to reflect its primary effect in this engine.

The primary goals:

1. Paint blending mimics different aspects of physical media, giving as much control as possible to artists
2. Paint Thickness value has similar range of response as Lightness Strength in pixel engine at 100 Color Rate
3. Each primary smudging control (Color Rate, Smudge Length, and in this case Paint Thickness) works as independently as possible, and has an expected effect on the final product, while not violating the next goal:
4. Paint blending mostly prevents undesirable digital effects, such as runaway lightness buildup, or canvas color being moved but its associated height staying behind

I can confirm #2 is met (each mode appears identical to pixel engine at 100 Color Rate), and believe the others are met as well, though will need more testing to confirm. Each mode has its limitations to prevent specific problems/pitfalls, but together allow almost all possible desired outcomes.

I'll add more specific descriptions of the different modes and their specific goals/features and limitations later...