Sculpt mode in Grease Pencil should not use the origin point of
`DrawingPlacement` and only apply the plane projection to relative
offsets.
A new variant of the `foreach_editable_drawing` function is introduced
that provides a `DeltaProjectionFunc` instead of `DrawingPlacement`.
This projection callback takes a layer-space position and a screen-space
delta and returns a layer-space position with the constrained delta
applied. The projection function is based on the tools settings'
"lock axis", like `DrawingPlacement`, but ignores the origin point.
Exception is the smoothing tool, which does not restrict movement along
the lock axis. This can be implemented if desired, but for a
non-directional tool it seems the lock axis isn't very meaningful.
Pull Request: https://projects.blender.org/blender/blender/pulls/128022
cbe2bb6755