samples: fix python samples compatibility with demo.py

samples/python/stereo_match.py

@@ -33,7 +33,7 @@ def write_ply(fn, verts, colors):
         np.savetxt(f, verts, fmt='%f %f %f %d %d %d ')
 
 
-if __name__ == '__main__':
+def main():
     print('loading images...')
     imgL = cv.pyrDown(cv.imread(cv.samples.findFile('aloeL.jpg')))  # downscale images for faster processing
     imgR = cv.pyrDown(cv.imread(cv.samples.findFile('aloeR.jpg')))
@@ -75,4 +75,11 @@ if __name__ == '__main__':
     cv.imshow('left', imgL)
     cv.imshow('disparity', (disp-min_disp)/num_disp)
     cv.waitKey()
+
+    print('Done')
+
+
+if __name__ == '__main__':
+    print(__doc__)
+    main()
     cv.destroyAllWindows()