Invert only within Free-Form Select's polygon

It turns out this isn't actually how MS Paint works (in Windows 98, my go-to reference), but, isn't it better this way?

README.md

@@ -179,7 +179,6 @@ To preview ANSI art files in file managers like Nautilus, Thunar, Nemo, or Caja,
 - Some languages don't display correctly.
 - Large files can make the program very slow, as can magnifying the canvas. There is a 500 KB limit when opening files to prevent it from freezing.
 - Free-Form Select stamping/finalizing is incorrect when the selection is off-screen to the left or top.
-- Image > Invert Colors ignores Free-Form Select's mask region, and acts on the rectangular bound.
 - Moving the selection with the arrow keys does not cut out the selection from the canvas, it only moves the selection box.
 - Status bar description can be left blank when selecting a menu item <!--I'm guessing Leave can come after close-->
 - Menu items like Copy/Cut/Paste are not grayed out when inapplicable. Only unimplemented items are grayed out.

src/textual_paint/paint.py

@@ -3553,20 +3553,11 @@ Columns: {len(palette) // 2}
             if sel.textbox_mode:
                 return
             if sel.contained_image is None:
-                # TODO: DRY undo state creation (at least the undos/redos part)
-                # FIXME: handle Free-Form Select's mask
-                action = Action(_("Invert Colors"), sel.region)
-                action.update(self.image)
-                if len(self.redos) > 0:
-                    self.redos = []
-                self.undos.append(action)
-
-                self.image.invert_region(sel.region)
-                self.canvas.refresh_scaled_region(sel.region)
-            else:
-                # No extra undo state if the selection is already cut out
-                sel.contained_image.invert()
-                self.canvas.refresh_scaled_region(sel.region)
+                self.extract_to_selection()
+                assert sel.contained_image is not None
+            # Note: no undo state will be created if the selection is already extracted
+            sel.contained_image.invert()
+            self.canvas.refresh_scaled_region(sel.region)
         else:
             # TODO: DRY undo state creation (at least the undos/redos part)
             action = Action(_("Invert Colors"), Region(0, 0, self.image.width, self.image.height))