Fix Open/Save As dialog starting location when file opened with relative path

The directory tree wasn't expanded because it was searching for e.g.
LICENSE.txt at the root of the filesystem (or perhaps AS the root?)

paint.py

@@ -1178,7 +1178,7 @@ if args.filename:
     # else:
     with open(args.filename, 'r') as my_file:
         app.image = AnsiArtDocument.from_text(my_file.read())
-        app.filename = args.filename
+        app.filename = os.path.abspath(args.filename)
 if args.clear_screen:
     os.system("cls||clear")
 if args.theme not in ["light", "dark"]: