Currently, brushes only have an ##alpha## property with a (0, 10) range
that was exposed in the UI as "Strength". This was causing multiple
issues:
- 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