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.