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 Lanthony (yann-lty)