Instead of requiring `GreasePencilStrokeParams` to have a
`DrawingPlacement`, pass it to the `foreach_editable_drawing`
callback.
This makes it possible to have a variant of `foreach_editable_drawing`
that doesn't use a `DrawingPlacement`.
The `Smooth`, `Thickness`, and `Strength` sculpt
brushes don't need this for example.
Also in preparation of using `GreasePencilStrokeOperationCommon`
in more places like vertex painting and weight painting.
Pull Request: https://projects.blender.org/blender/blender/pulls/125903
656bbd4c7c