Fixed the bug with random offset selection

When you wish to mirror the image you are also forced to set the crop_size param, which is fine. But if you set the crop_size to be equal to the width (or height) to cancel its behaviour, the program will terminate without any explanations.

The problem is here
```c++
h_off = Rand() % (height - crop_size);
w_off = Rand() % (width - crop_size);
```

This bugfix sets the h_off or w_off to be zero where appropriate.

Let's make it clear (one way or another) that this function draws an _integer_ from `Uniform({0, ..., n-1})`, rather than something from a continuous interval which `[0, n)` will evoke to some.