spirv-builder could show warnings from building shaders.

[eddyb]

Cargo build scripts have the ability to produce output through Cargo via cargo:warning=MESSAGE even when successful.

We should probably make spirv-builder transform stderr lines from its nested cargo build (of the shaders) into cargo:warning, but we may need to use the JSON output mode to be able to tell apart Cargo output from rustc output (as we presumably don't want to dump Compiling ... messages as "warnings").

We already pass --message-format=json-render-diagnostics to the nested cargo build though (to parse out "artifact notifications"), so it should be pretty easy to further rely on the JSON output.