Clean up Tk root widget

I was hoping this would fix the Python rocket icon showing in the dock on macOS, but it doesn't. It still stays until the program exits.
Might still be good to do though.

src/textual_paint/paint.py

@@ -3618,7 +3618,9 @@ Columns: {len(palette) // 2}
             import tkinter
             root = tkinter.Tk()
             root.withdraw()
-            return root.winfo_screenwidth(), root.winfo_screenheight()
+            size = root.winfo_screenwidth(), root.winfo_screenheight()
+            root.destroy()
+            return size
         except Exception as e:
             print("Failed to get screen size:", e)
             return 1920, 1080