Show warnings generated by glslang

[tomaka]

Once rust-lang/cargo#2630 lands

[rukai]

glsl-to-spirv is no more. #947

[nicokoch]

Could warnings be shown regardless? shaderc uses glsl internally.
Or are they shown already? Haven't really used vulkano recently.

[rukai]

Good point, in fact, shaderc makes it really easy to get. https://docs.rs/shaderc/0.3.11/shaderc/struct.CompilationArtifact.html#method.get_warning_messages

[Rua]

The feature linked by Tomaka isn't actually relevant here, because Vulkano-shaders is a procedural macro rather than a build script. Emitting warnings from a procedural macro is possible via the proc_macro_error crate, but this is not possible on stable Rust yet because it's blocked by this feature.