SDL Frontend: Add shared context support

src/yuzu_cmd/emu_window/emu_window_sdl2.h

@@ -27,6 +27,8 @@ public:
     /// Releases the GL context from the caller thread
     void DoneCurrent() override;
 
+    std::unique_ptr<Core::Frontend::GraphicsContext> CreateSharedContext() const override;
+
     /// Whether the window is still open, and a close request hasn't yet been sent
     bool IsOpen() const;