Add root_path_env property to shaders macro

[Firestar99]

In order to load a shader generated by the build script one would need to specify the path from the Cargo.toml into the target directory, but one may not even now where exactly the target directory is located. It may be ./target on a single crate, ../target or ../../target in a workspace environment or even in a totally different folder if the crate is used as a library from some other cargo project. As src/bytes are required to be string symbols one cannot specify them to concat!(env!("OUT_DIR"), "path.spv"), so with this PR I introduce the root_path_env property to the shader! macro.

//! ## `root_path_env: "..."`
//!
//! Instead of searching relative to your `Cargo.toml`, search relative to some other folder
//! specified by this env variable. The intended use case is using `OUT_DIR` to be able to load
//! shaders generated by your build script. Defaults to `CARGO_MANIFEST_DIR` corresponding to the
//! folder of your `Cargo.toml`.

For testing purposes I implemented this into my rust-gpu integration example project in the branch root_path_env_pr now in master:
https://github.com/Firestar99/rust-gpu-vulkano-example/

1. Update documentation to reflect any user-facing changes - in this repository.

2. Make sure that the changes are covered by unit-tests.

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Changelog:

### Additions
- add `root_path_env` property to allow loading shaders generated by a build script

[Firestar99]

I'm not exactly sure how to go about adding unit tests. I could of course add a build script that copies an glsl file into the build dir and then use it in a test, but adding that to the shaders crate directly would also cause everyone using that crate to execute that unnecessary build script. I could also add a new crate just for testing this but that also seems a little overkill.
One could add such a "test" if you were to add rust-gpu examples to your repo, but that would also bind your repo to the relatively large rust-gpu stack whenever you'd compile an example.

[marc0246]

I don't think that this can be unit tested. It would need an integration test or a whole system test (like an example, which you mentioned) but that would be total overkill for a change like this I think. There's only so much that can go wrong with a small change like this so I wouldn't worry about it personally.

[marc0246]

I like the name for the field you ended up with! It's very descriptive. Everything works for me, just a couple changes I would like.

[Firestar99]

I just applied all your suggestions, don't really have anything to add to them

[Firestar99]

Would you deem it valuable to add documentation that one can specify custom rustc env vars in the build script?

build.rs

let out_dir = env::var("OUT_DIR").unwrap();
let shader_out_dir = format!("{out_dir}/../../../../spirv-builder/spirv-unknown-spv1.3/release/deps/example_shader.spvs/");
println!("cargo:rustc-env=SHADER_OUT_DIR={shader_out_dir}");

shader include:

vulkano_shaders::shader! {
	ty: "vertex",
	root_path_env: "SHADER_OUT_DIR",
	bytes: "image_shader-image_vs.spv",
}

from Firestar99/rust-gpu-vulkano-example@2d27af7

[marc0246]

Sure, if you want to add that I think that would be great.

[marc0246]

Awesome, thank you for the work!