For the "Sphere" 3D brushes, the depth and radius are only sampled at
the beginning of the stroke. This didn't work when tablet pressure is
used as a factor for the brush radius. Now the 3D brush depth is found
with the max radius (as if the pressure was 1.0), but the pressure
factor is used afterwards.
Restructuring the way the brush executors stored the radius made
the change a bit clearer, which is where most of the diff comes from.
Differential Revision: https://developer.blender.org/D15002
8f3847aef3