The GLSL shader source is not compiled as part of the normal cargo build process. This was a conscious decision sparked by the current state of the ecosystem; compiling GLSL-to-SPIR-V requires a C++ toolchain including CMake, which is an unacceptable constraint for a simple crate providing a pixel buffer.
If you need to modify the GLSL sources, you must also recompile the SPIR-V as well. This can be done with `glslang`, `glslc`, etc.