Added a safer constructor for Image

[eliPerez12]

This PR is the solution to issue #634.

I've added a new constructor Image::new(width, height) -> Image , which abstracts away the process of making a new image, making it less error prone. Having to manually manage the amount of bytes needed was unnecessary and inefficient.

This is my first ever pull request so let me know if i missed anything or need to improve on anything.

[asmodehn]

Just jumping in here since I encountered issues with image manipulation for a small project:

We probably want to check overflows of usize type here. see for instance https://docs.rs/image/latest/src/image/buffer.rs.html#1271