Bugfix: Duplicating Grease Pencil datablocks wasn't doing so in a safe way

Grease Pencil data (bGPdata) is now a datablock, so it isn't safe to use
MEM_dupallocN() for copying new instances of these anymore.

source/blender/blenkernel/intern/gpencil.c

@@ -286,7 +286,7 @@ bGPdata *gpencil_data_duplicate(bGPdata *src)
 		return NULL;
 	
 	/* make a copy of the base-data */
-	dst = MEM_dupallocN(src);
+	dst = BKE_libblock_copy(&src->id);
 	
 	/* copy layers */
 	BLI_listbase_clear(&dst->layers);