Disable the output when calculating optical flow in OpticalFlow.cpp

[thkkk]

When I call the pyflow.coarse2fine_flow function, the program will output a lot of:

Constructing pyramid...done!
Pyramid level 7
Pyramid level 6
Pyramid level 5
Pyramid level 4
Pyramid level 3
Pyramid level 2
Pyramid level 1
Pyramid level 0

which completely covers my normal log information. Therefore, it is necessary to set OpticalFlow::IsDisplay to false in OpticalFlow.cpp before building.