Building on WASM

glslang needs a sysroot that isn't available for wasm32-unknown-unknown

`wasm32-wasip1` works though:

```
$ RUSTFLAGS="--cfg=web_sys_unstable_apis --Z wasm_c_abi=spec" WASI_SYSROOT="/usr/share/wasi-sysroot/" cargo build --target=wasm32-wasip1
```