Make shaderc output independent of backend enums (#2411)

* Fix typo in SPIR-V id

* Use backend-agnostic shaderc output

...as opposed to directly storing and comparing against Vulkan and WebGPU enums. This is backwards-compatible with existing code and shaders.

* Remove comments and Undefined value

src/amalgamated.cpp

@@ -20,6 +20,7 @@
 #include "renderer_nvn.cpp"
 #include "renderer_vk.cpp"
 #include "renderer_webgpu.cpp"
+#include "shader.cpp"
 #include "shader_dx9bc.cpp"
 #include "shader_dxbc.cpp"
 #include "shader_spirv.cpp"