samples: fix python samples compatibility with demo.py

samples/python/texture_flow.py

@@ -16,7 +16,7 @@ from __future__ import print_function
 import numpy as np
 import cv2 as cv
 
-if __name__ == '__main__':
+def main():
     import sys
     try:
         fn = sys.argv[1]
@@ -45,3 +45,11 @@ if __name__ == '__main__':
     cv.imshow('input', img)
     cv.imshow('flow', vis)
     cv.waitKey()
+
+    print('Done')
+
+
+if __name__ == '__main__':
+    print(__doc__)
+    main()
+    cv.destroyAllWindows()