Add Buffer::pixel_rows and Buffer::pixels_iter iterators

[madsmtm]

Add:

impl Buffer<'_> {
    pub fn pixel_rows(&mut self) -> impl Iterator<Item = &mut [u32]> + DoubleEndedIterator + ExactSizeIterator;
    pub fn pixels_iter(&mut self) -> impl Iterator<Item = (u32, u32, &mut u32)> + DoubleEndedIterator;
}

pixel_rows is a prerequisite for #291, as using a buffer with stride will be quite cumbersome without them. It currently chunks on the width, but should chunk on the stride once we add that. pixels_iter is more of a useful shorthand, it's not as critical, but still be nice to have.

I decided to return opaque impl Iterator<...>, we can add an explicit BufferRows<'_> iterator type later if desirable.

[MarijnS95]

I hope that we can soon drive this from the buffer instead, taking a stride into account.

[madsmtm]

Working on it ;)

[MarijnS95]

Great way to get rid of the NonZero::get() API instead.

[madsmtm]

It's not really gone though, people will wanna use it in larger examples, see e.g. examples/rectangle.rs (if you want to calculate distance a given pixel is from the right or bottom edge).

[madsmtm]

I renamed iter_pixels -> pixels_iter, I felt that was more in line with how I've seen iterator methods named elsewhere (e.g. par_iter).