This ports the `lock_material` layer property to GPv3.
For this, the `layer_index` is also required in `retrieve_editable_points`
and `retrieve_editable_strokes` to choose strokes based on
the `lock_material` property.
In `retrieve_editable_elements` we now pass the `MutableDrawingInfo`
to get both the `drawing` and `layer_index`.
Pull Request: https://projects.blender.org/blender/blender/pulls/119913
c80e30684c