Compute min_resolution in prepare_image_inputs
# What does this PR do?

If `feature_extract_tester.min_resolution` is specified, the images have to be at least that large, otherwise we will get image width and/or height `0` and it gives error.

An error is [here](https://github.com/huggingface/transformers/runs/7071766841?check_suite_focus=true):
```
>       return self._new(self.im.resize(size, resample, box))
E       ValueError: height and width must be > 0
```


So far, we have the following in `GLPNFeatureExtractionTester` and other testers
```
        min_resolution=30,
        ...
        size_divisor=32,
```

issue spotted by @Rocketknight1 , thanks!
_The documentation is not available anymore as the PR was closed or merged._