Support include directives on shaderc

[fand]

Hello 👋 Thanks for great library!!

I was tweaking around notan_macro to see if it's possible to support #include.
Then I found shaderc can handle #include directives by passing an additional option.

So, this PR adds an option passed to shaderc to include shader files in following format:

• Relative includes #include "foo/bar.glsl"
  • This tries to read "foo/bar.glsl" relative to the input shader file.
• Standard includes #include <foo/bar.glsl>
  • This tries to read "foo/bar.glsl" under the root directory (CARGO_MANIFEST_DIR or .)

Also I added examples/shaders/draw_shader_include.glsl, an example file to test #include.
You can run it by cargo run --example draw_shader_include --features shaderc.

NOTE: include directive only works on include_fragment_shader! / include_vertex_shader! macros, not on fragment_shader! / vertex_shader! because it's difficult to determine the directory to search for the include files.

[Nazariglez]

This is great, thank you so much!

[Nazariglez]

This is failing because shaderc is disabled on the build system by default, mainly because docs were not building for crates, so I need to check how to enable it only for the build system. Another option is avoid the compilation of the example if shaderc is not enabled. I'll take a look today.

[Nazariglez]

@fand is it ok to you if we remove the example? Only the example.

I wasn't able to find a way to exclude the example from the cargo test command, it seems that cargo itself have some issues open related to this. The only options I can see is to enable shaderc on the ci machine, however I am not a big fan of this solution, because it will try to compile it on each build, and is a big dependency. So I am more inclined to just remove the example, because is a niche feature.

What do you think?

[fand]

Thanks for review!

So I am more inclined to just remove the example, because is a niche feature.

Yeah I totally agree 👍 I removed the example

it seems that cargo itself have some issues open related to this

Just curious, could you give me the link of the issue if you have?

[Nazariglez]

This is the issue I found: rust-lang/cargo#7233 they are trying to avoid the compilation using a config flag (I did it with not(feature=shaderc)), however this doesn't work. Using cargo test --workspace --exclude my_example does not work either. That's why I thought about to remove the example.

Sorry for the inconveniences and thank you so much for the PR!