Add pdsg (palladium spirv-generator)

- remove glslang from pd-system
- use spirv blobs instead of glsl 460
- Use Legacy shaders for OpenGL2 backend
- add devnotes

backends/include/pd_system/gfx_opengl2.hpp

@@ -39,5 +39,7 @@ class GfxOpenGL2 : public GfxDriverBase<GfxOpenGL2Config> {
   int pLocTex = 0;
   int pLocAlfa = 0;
   int pLocProjection = 0;
+  static const char* pVertCode;
+  static const char* pFragCode;
 };
 }  // namespace PD