* This will allow the backend to specify both the native format (e.g. SPIR-V,
DXBC) as well as a language it might be able to internally compile (GLSL or
HLSL).
* The caller will then able to decide for itself whether it wants to compile to
native format and pass that down, or pass the language down and let it be
built internally.
* Currently BuildTargetShader still only accepts shader source.