Fix: Sub-panel backdrop sometimes draws when closed

My last cleanup commit for this function missed this case. It likely
happens because the panel's block size doesn't update properly somewhere.
Short of investigating that right now, it makes sense to return early
in this case anyway.

source/blender/editors/interface/interface_panel.c

@@ -1166,6 +1166,10 @@ static void panel_draw_aligned_backdrop(const Panel *panel,
   const bool is_subpanel = panel->type->parent != NULL;
   const bool is_open = !UI_panel_is_closed(panel);
 
+  if (is_subpanel && !is_open) {
+    return;
+  }
+
   const uint pos = GPU_vertformat_attr_add(
       immVertexFormat(), "pos", GPU_COMP_F32, 2, GPU_FETCH_FLOAT);