Caused by parallel access to static clipboard storage: Multiple threads
could end up calling `ensure_grease_pencil_clipboard()` in parallel.
Here this resulted in a surprisingly reliable memory leak.
Use a simple mutex lock to prevent unsynchronized access.
Steps to reproduce were:
- Add Grease Pencil object
- Enter grease pencil Sculpt mode
- Select Clone brush asset
- Click in 3D Viewport
- Quit Blender (see console log for leak reports)
Pull Request: https://projects.blender.org/blender/blender/pulls/125428
3dfd9d84eb