Image api improvements

[hydra]

While working with the img API I was frustrated to find that I had to create a vector of u8's even though I already had either a DynamicImage instance or PathBuf instance. e.g. when creating a new file (DynamicImage) or opening a file (PathBuf).

I also noted that img_dynamic visibility is pub(crate) which meant I could not use that either.

This PR adds three well-named methods (commit 1) and deprecates the old one (commit 2).

See commits/changes.