When removing a modifier, changing the layer transform or updating
the parent of a grease pencil object that has a multi-user datablock
and animation data, the eval data is not updated properly (after a
frame change). This can also cause memory leaks.
The fix makes sure that we free and reset any runtime copy
(`ob->runtime.gpd_eval`) in `BKE_gpencil_prepare_eval_data`.
Note: As far as we can tell, `ob->runtime.gpd_orig` is unused and could
be removed. The assignment in `BKE_gpencil_prepare_eval_data`
seemed to be unnecessary.
Co-authored-by: @yann-lty
Reviewed By: antoniov
Maniphest Tasks: T96145
Differential Revision: https://developer.blender.org/D14236
e8dc82311d