Forward rustc warnings

[Firestar99]

• replace enum MetadataPrintout with struct BuildScriptConfig for more flexibility
  • Primarily: spirv_builder.build_script.defaults allows us to add new options like below in the future
  • MetadataPrintout::DependencyOnly is now spirv_builder.build_script.dependency_info = Some(true) and defaults to spirv_builder.build_script.defaults (property above)
  • MetadataPrintout::Full is now spirv_builder.build_script.env_shader_spv_path = Some(true) and defaults to false (preventing conflicts with multimodule)
• forward rustc warnings and errors as build script warnings (via cargo::warning={})
  • includes support for ansi colors!

Example error

Image cause github doesn't support ansi in code blocks. Also note how in the shader warning, the final "space-engine-shader (lib) generated 1 warning" is missing. This is due to cargo --quiet also suppressing those, next to the other status messages.

close #346
close Rust-GPU/cargo-gpu#103

[Firestar99]

Since I'm already changing this part, I've considered adding some more ways to "send" the shader binaries from the build script into the source code, like multimodule support for env vars or better source code gen. But I feel like exploring that would be better in a followup PR.

[Firestar99]

Note: the code is outdated, I decided to go back on renaming the env var to SHADER_SPV_PATH since it breaks the android build

[LegNeato]

Would it make sense to split builder into two public crates, one for using in build.rs and one for using from the CLI / other tools?

[Firestar99]

I don't think we should split spirv-builder like that. cargo-gpu being built on top may also be used from different use-cases, and would suddenly need to use two different libraries.

But what does make sense is splitting cargo-gpu into two crates, but differently: One cargo-gpu with all the cli goodness with all it's dependencies, specifically clap. The other cargo-gpu-install that just has the install action, since that's the only thing really needed for build scripts and watch()-ing binaries, and no cli deps. See this issue and the rejected PR1 and PR2. EDIT: here's my attempt at it: Rust-GPU/cargo-gpu#132

Setting a single bool when used from a build script is far simpler, just use whatever method you found in another project, and make sure you adjust this one value to switch between build script and watch in your binary mode. Though I do agree slightly error-prone since it's easy to miss.