Change images data field from Vec<u8> to Cow<'static, [u8]>

## What problem does this solve or what need does it fill?
Distributing single binaries is really convenient and somnetimes a very efficient solution. The core question is wether the data field actually has to be a `Vec` or if it could be replaced with a `Cow` which spares memory allocation if the data is already inside the binary.

## What solution would you like?
Optimally one would replace the data field with a `Cow<'static, [u8]>` and fits other functions that need to mutate the buffer to use `Cow::to_mut`

## What alternative(s) have you considered?
This seems to be a rather "niche" case but it could add a lot of value, espacially for "default"/fallback textures.

## Additional context
Generally loading Assets without the `AssetServer` is quite a hassle in my opinion, the documentation on how to create the assets could be improved or there could be a embedded assets asset server io based on `rust_embed`, but then there would also need to be an easy way to swap the `AssetIo` of the `AssetServer`.

Start by locating the images data field and every function that reads or mutates its buffer. Review the default or fallback texture paths and the issue's embedded-asset context; done means the representation supports borrowed static data while mutation paths still work without breaking image loading.

Written by the indexing model from the issue text.