- Add ShaderLanguage enum (HLSL, GLSL, SPIRV)
- Extend ShaderCompileDesc with source/sourceLanguage fields
- D3D12Device::CompileShader supports both file and embedded source
- OpenGLDevice::CompileShader supports embedded GLSL source
- Refactor test_shader.cpp to use embedded source for both backends
This enables consistent shader compilation across D3D12 and OpenGL
backends while maintaining backend-specific shader language support.