de8ce4e34b
Adds a user count to drawings to track how many frames use the drawing. If the user count hits `0` the drawing should be deleted. Also adds a function to delete a frame in a layer. This will decrement the user count of the drawing and delete it if there are no users left. This is consistent with the way GPv2 worked. Pull Request: https://projects.blender.org/blender/blender/pulls/110579