Fix: GPv3: Merge layers operator did not copy grease pencil paramters

The merge layer operator did not copy the parameters of the original
Grease Pencil meaning that it would e.g. loose onion skinning settings.
This fixes the issue by making sure to copy the parameters.

source/blender/editors/grease_pencil/intern/grease_pencil_layers.cc

@@ -792,6 +792,7 @@ static int grease_pencil_merge_layer_exec(bContext *C, wmOperator *op)
   }
 
   GreasePencil *merged_grease_pencil = BKE_grease_pencil_new_nomain();
+  BKE_grease_pencil_copy_parameters(grease_pencil, *merged_grease_pencil);
   ed::greasepencil::merge_layers(
       grease_pencil, src_layer_indices_by_dst_layer, *merged_grease_pencil);
   BKE_grease_pencil_nomain_to_grease_pencil(merged_grease_pencil, &grease_pencil);