error excerpts require execution from the project root directory

[RayMarch]

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when pipeline encoding errors are triggered, they normally are displayed with an error excerpt. e.g. if we modify the api_showcase example to trigger an error and use

cargo run --bin api_showcase

the following output is generated

shame encoding error:
  --> examples/api_showcase/src/main.rs:211:26
    |
210 |    // Rasterization gives access to the fragment-stage api via `frag`
211 >    let frag = primitive.rasterize_multisample([1].into() ,sm::Accuracy::Relaxed);
    |                         ^
    |
multisample/supersample rasterization requires a sampling mask with at least 2 samples. Only 1 sample(s) was/were requested

however if the current directory is not the project root, e.g. because we navigate into the inner crate's folder

cd examples/api_showcase
cargo run

the following output is generated

shame encoding error:
  --> examples/api_showcase/src/main.rs:211:26
[code excerpt not found]
multisample/supersample rasterization requires a sampling mask with at least 2 samples. Only 1 sample(s) was/were requested

because reading the file examples/api_showcase/src/main.rs fails from this directory.

potential solutions are

1. add an interface for the user to specify a prefix (e.g. ../../)
2. if reading the file fails, check if the std::env::current_dir() contains parts of the error path, subtract that overlap and try reading again
3. accept this as the behavior of error excerpts