Grease Pencil v3 did not have crazyspace support yet. Without this sculpting on
deformed geometry (e.g. on top of an armature modifier) will yield incorrect
offsets because the tool writes to original data based on deformed positions.
This patch adds computation of local deformation matrices which are stored in
the `GeometryComponentEditData`. Those matrices are then used to convert local
deformation of the evaluated geometry back to a deformation of the original
geometry. All the relevant sculpt tools support the crazyspace correction now,
using the `compute_orig_delta` helper function.
Computing the deformation matrices should happen alongside modifier evaluation
for any deforming modifier. This has been implemented for the armature modifier,
others can be added.
A fallback implementation for curves could also be added for modifiers that
don't have an easy way to calculate local transformation. A "natural"
orientation for both the original and deformed positions is calculated, then the
difference yields deform matrices. For meshes the approach is to use the surface
normal and a stable tangent space. For curves the common local coordinate frame
based on parallel transport might be used.
Currently crazyspace correction for the _Clone_ tool does not work because of
#131496.
Pull Request: https://projects.blender.org/blender/blender/pulls/131499
0d8f040c8b