Pcbnew: polygon booleans - try subtraction in both orders if null result

While there is technically some kinds of defined order (last selected item is
'A' in the 'A - B (- C...)' operation, it's easy to drag select and then
get a null result.

So if this happens to a non-commutative operation, try again in the
reverse order. In the usual case (2 items), this will get what the user,
presumably, wants. If there are more than 2, it will either work, or the
user can undo and try merging the "B" operand before subtracting, or do
them one at a time (at which point the order-reverse logic will help
again).

Thanks to Kliment for an this usability suggestion!

pcbnew/tools/edit_tool.cpp

@@ -1890,29 +1890,62 @@ int EDIT_TOOL::BooleanPolygons( const TOOL_EVENT& aEvent )
     // Construct an appropriate routine
     std::unique_ptr<POLYGON_BOOLEAN_ROUTINE> boolean_routine;
 
-    if( aEvent.IsAction( &PCB_ACTIONS::mergePolygons ) )
+    const auto create_routine = [&]() -> std::unique_ptr<POLYGON_BOOLEAN_ROUTINE>
     {
-        boolean_routine = std::make_unique<POLYGON_MERGE_ROUTINE>( frame()->GetModel(), change_handler );
-    }
-    else if( aEvent.IsAction( &PCB_ACTIONS::subtractPolygons ) )
-    {
-        boolean_routine = std::make_unique<POLYGON_SUBTRACT_ROUTINE>( frame()->GetModel(), change_handler );
-    }
-    else if( aEvent.IsAction( &PCB_ACTIONS::intersectPolygons ) )
+        // (Re-)construct the boolean routine based on the action
+        // This is done here so that we can re-init the routine if we need to
+        // go again in the reverse order.
+
+        BOARD_ITEM_CONTAINER* const model = frame()->GetModel();
+        wxCHECK( model, nullptr );
+
+        if( aEvent.IsAction( &PCB_ACTIONS::mergePolygons ) )
+        {
+            return std::make_unique<POLYGON_MERGE_ROUTINE>( model, change_handler );
+        }
+        else if( aEvent.IsAction( &PCB_ACTIONS::subtractPolygons ) )
+        {
+            return std::make_unique<POLYGON_SUBTRACT_ROUTINE>( model, change_handler );
+        }
+        else if( aEvent.IsAction( &PCB_ACTIONS::intersectPolygons ) )
+        {
+            return std::make_unique<POLYGON_INTERSECT_ROUTINE>( model, change_handler );
+        }
+        return nullptr;
+    };
+
+    const auto run_routine = [&]()
     {
-        boolean_routine = std::make_unique<POLYGON_INTERSECT_ROUTINE>( frame()->GetModel(), change_handler );
-    }
-    else
+        // Perform the operation on each polygon
+        for( PCB_SHAPE* shape : items_to_process )
+            boolean_routine->ProcessShape( *shape );
+
+        boolean_routine->Finalize();
+    };
+
+    boolean_routine = create_routine();
+
+    wxCHECK_MSG( boolean_routine, 0, "Could not find a polygon routine for this action" );
+
+    // First run the routine and see what we get
+    run_routine();
+
+    // If we are doing a non-commutative operation (e.g. subtract), and we just got null,
+    // assume the user meant go in the opposite order (this is probably mainly useful
+    // with a selection size of 2 items)
+    if( !boolean_routine->IsCommutative() && items_to_select_on_success.empty() )
     {
-        wxASSERT_MSG( false, "Could not find a polygon routine for this action" );
-        return 0;
-    }
+        // Clear the commit and the selection
+        commit.Revert();
+        items_to_select_on_success.clear();
 
-    // Perform the operation on each polygon
-    for( PCB_SHAPE* shape : items_to_process )
-        boolean_routine->ProcessShape( *shape );
+        // Reverse the order of the shapes and try again
+        std::reverse( items_to_process.begin(), items_to_process.end() );
 
-    boolean_routine->Finalize();
+        // Run the routine again
+        boolean_routine = create_routine();
+        run_routine();
+    }
 
     // Select new items
     for( BOARD_ITEM* item : items_to_select_on_success )

pcbnew/tools/item_modification_routine.h

@@ -343,6 +343,11 @@ public:
     {
     }
 
+    /**
+     * @brief False if the order of the polygons matters
+     */
+    virtual bool IsCommutative() const = 0;
+
     void ProcessShape( PCB_SHAPE& aPcbShape );
 
     /**
@@ -380,6 +385,8 @@ public:
     {
     }
 
+    bool IsCommutative() const override { return true; }
+
     wxString                GetCommitDescription() const override;
     std::optional<wxString> GetStatusMessage() const override;
 
@@ -396,6 +403,8 @@ public:
     {
     }
 
+    bool IsCommutative() const override { return false; }
+
     wxString                GetCommitDescription() const override;
     std::optional<wxString> GetStatusMessage() const override;
 
@@ -412,6 +421,8 @@ public:
     {
     }
 
+    bool IsCommutative() const override { return true; }
+
     wxString                GetCommitDescription() const override;
     std::optional<wxString> GetStatusMessage() const override;