added comments in gpu.hpp

This was crashing my code:
    for( size_t i = 1; i < imagePoints.size(); i++ )
        objectPoints.push_back(objectPoints[0]);

Changed by:
objectPoints.resize(imagePoints.size(),objectPoints[0]);

Plus clean up on Qt code

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avoid extra gpu memory allocation in BP and CSBP

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  fixed some bugs in init_data_cost on first level (added non-reduction version for first level)
  optimized compute_data_cost like init_data_cost (used reduction scheme)
  avoid temp matrix

some refactoring of StereoBeliefPropagation.