Currently, brushes only have an alpha property with a (0, 10) range
that was exposed in the UI as "Strength". This was causing multiple
- The sculpt mode was using a property named alpha and BKE_brush_alpha functions to calculate the brush strength that scales the displacement of most brushes, which was confusing
- When dealing with painting tools, the painting color could have an alpha of 10 when using BKE_brush_alpha_get, which needs to be handled as part of the color mixing code to avoid bugs.
- As brushes that have displacement and color at the same time are planned (for sculpt vertex colors and future texture painting/normal editing painting modes) it makes sense to have the sculpt displacement strength and the alpha as separate properties