Added support to get all pixels(x,y,r,g,b) or specfic pixel.

[WilgnerFSDev]

I added two functions to get all or individual pixels of the image returning the position and the RGB.
I also updated the readme with the 2 examples (all pixels and a specific pixel).

I hope you see the benefit and believe it's an improvement. Any questions please contact me and feel free to edit the code (today is my first day with rust so suggestions for improvement are very welcome).

[nashaofu]

Maybe storing a copy of the original RGBA value is better，it is easy to do other processing on the image without decode, such as cropping.

pub struct Image {
  width: u32,
  height: u32,
  buffer: Vec<u8>,
  rgba: Vec<u8>, // The original rgba data of the picture
}

get_pixel also easy to implement.

pub fn get_pixel(&self, x: u32, y: u32) -> Pixel {
  let index = y * self.width + x;
  Pixel {
    x,
    y,
    r: self.rgba.get(index),
    g: self.rgba.get(index + 1),
    b: self.rgba.get(index + 2),
    a: self.rgba.get(index + 3),
  }
}

[WilgnerFSDev]

In my project I need all pixels (positions and rgb) so I can pass the pixels to my algorithm.
I have stored it in a list in case in the future someone wants to do other types of operations.

Before editing, you commented if it is really necessary to add this feature, in fact it is something very specific for me, I decided send a pull request in case someone is going through the same thing or a similar situation to mine. Feel free to accept or not, since is really specific situation, anyways thanks for you time and by the library, it's really useful for me.

[c-h-johnson]

This is now added in #69.

Relevant methods:

• https://docs.rs/image/latest/image/struct.ImageBuffer.html#method.enumerate_pixels
• https://docs.rs/image/latest/image/struct.ImageBuffer.html#method.pixels
• https://docs.rs/image/latest/image/struct.ImageBuffer.html#method.get_pixel