Fixed division zero and unintentional bias in Otsu's threshold (CUDA)

[troelsy]

This PR contains two bug fixes to the CUDA implementation of Otsu:

1. You could have a histogram, where n_samples_above or n_samples_below could be zero. In that case we would do a zero division
2. Assuming threshold_sums[0] contains the full sum of the histogram is not true as it only contains the sum of bins above the threshold (0). I changed n_samples to be a parameter, so we can use src.rows * src.cols instead.

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