Add SpirvHelper (Shader cross compilation)

- Make OpenGL2 and OpenGL3 use the same base shaders (using SpirvHelper)
- Add Transpose func to Mat4

backends/include/pd_system/gfx_opengl3.hpp

@@ -39,7 +39,5 @@ class GfxOpenGL3 : public GfxDriverBase<GfxOpenGL3Config> {
   int pLocTex = 0;
   int pLocAlfa = 0;
   int pLocProjection = 0;
-  static const char* pVertCode;
-  static const char* pFragCode;
 };
 }  // namespace PD