Only the evaluated positions were changed when deform or subsurf
modifiers were active. This was caused by only writing to the
evaluated positions owned by the PBVH and assuming these were
aliased with the mesh positions.
There were also quite a few TODOs in the area related to an extra
positions array that was passed around that was conceptually
unnecessary. Brush deformations should be based on the evaluated
positions, but only the original positions should be written to.
This PR resolves those TODOs while resolving sculpting with deform
modifiers and shape keys. We do this by only writing to the original
mesh positions. For shape keys we use the existing translations from
the brush rather than recomputing them, and only copy the mesh
original positions to the active shape key.
This also makes it easier to remove the PBVH's copy of vertex
positions in the future.
Pull Request: https://projects.blender.org/blender/blender/pulls/122842
e4ab6dd8de