Implementation of size_hint for ImageBuffer iterators should return the same as the ExactSizeIterator::len impl.

[GabrielDertoni]

The docs for std::iter::ExactSizeIterator state that

When implementing an ExactSizeIterator, you must also implement Iterator. When doing so, the implementation of Iterator::size_hint must return the exact size of the iterator.

however the current implementation of Iterator for the ImageBuffer iterators do not have such behaviour and use the default implementation that returns (0, None) even though these iterators do implement std::iter::ExactSizeIterator.

Expected

Every iterator that implements std::iter::ExactSizeIterator should also return the exact size in the size_hint function.

Actual behaviour

Implementations currently return the default (0, None).

Reproduction steps

The following test fails:

let image = RgbImage::from_raw(1, 1 vec![0; 3]).unwrap();
let iter = image.pixels();
let exact_len = ExactSizeIterator::len(&iter);
assert_eq!(iter.size_hint(), (exact_len, Some(exact_len)));