Supports the `OB_GREASE_PENCIL` type in the "Join Objects" operator.
Each object is merged into the active object one-by-one. This first
combines the layer trees of the source and destination, adds vertex
groups and materials, and appends the drawings to the destination array.
Then internal references are updated: drawing indices in frame data,
mask layer names, layer parents, material and vertex group indices.
The source curves are transformed into the target object space, so their
world position remains the same. Note that animation data is also
transferred to the new object, but __not__ transformed in case of
diverging object- or layer transforms. This is expected and handled the
same way by other object types.
Pull Request: https://projects.blender.org/blender/blender/pulls/128429
e07842de41