Use views to prevent image copies

[akaihola]

I was still intrigued by the problem of using views. I didn't manage managed to do it, but and in this incomplete implementation

• the brightest() function accepts images as GenericImage traits – both views and GrayImages should implement that
• views are passed into brightest() from the main program
• constant value pixels are returned since I couldn't figure out how to turn get_pixel() return values into Lumas

I can't wrap my head around solved this by using GenericImageView instead of GenericImage:

error[E0277]: the trait bound `&image::ImageBuffer<image::Luma<u8>, std::vec::Vec<u8>>: std::ops::DerefMut` is not satisfied
  --> src/main.rs:39:26
   |
14 | fn brightest<T: GenericImage>(img1: T, img2: T) -> GrayImage {
   |                 ------------ required by this bound in `brightest`
...
39 |         let top_pixels = brightest(orig, left);
   |                          ^^^^^^^^^ the trait `std::ops::DerefMut` is not implemented for `&image::ImageBuffer<image::Luma<u8>, std::vec::Vec<u8>>`
   |
   = help: the following implementations were found:
             <image::ImageBuffer<P, Container> as std::ops::DerefMut>
   = note: `std::ops::DerefMut` is implemented for `&mut image::ImageBuffer<image::Luma<u8>, std::vec::Vec<u8>>`, but not for `&image::ImageBuffer<image::Luma<u8>, std::vec::Vec<u8>>`
   = note: required because of the requirements on the impl of `image::GenericImage` for `image::SubImage<&image::ImageBuffer<image::Luma<u8>, std::vec::Vec<u8>>>`

[samoylovfp]

Wow! Kudos for figuring this out!