sdl: add check for GL extension support

src/yuzu_cmd/emu_window/emu_window_sdl2.h

@@ -46,6 +46,9 @@ private:
     /// Called when user passes the fullscreen parameter flag
     void Fullscreen();
 
+    /// Whether the GPU and driver supports the OpenGL extension required
+    bool SupportsRequiredGLExtensions();
+
     /// Called when a configuration change affects the minimal size of the window
     void OnMinimalClientAreaChangeRequest(
         const std::pair<unsigned, unsigned>& minimal_size) override;