Add a backend query to check shader encodings BuildTargetShader allows

* 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.