yuzu_tester: Add SDL2-based EmuWindow that doesn't show the window

src/yuzu_tester/emu_window/emu_window_sdl2_hide.h

@@ -0,0 +1,41 @@
+// Copyright 2019 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "core/frontend/emu_window.h"
+
+struct SDL_Window;
+
+class EmuWindow_SDL2_Hide : public Core::Frontend::EmuWindow {
+public:
+    explicit EmuWindow_SDL2_Hide();
+    ~EmuWindow_SDL2_Hide();
+
+    /// Swap buffers to display the next frame
+    void SwapBuffers() override;
+
+    /// Polls window events
+    void PollEvents() override;
+
+    /// Makes the graphics context current for the caller thread
+    void MakeCurrent() override;
+
+    /// Releases the GL context from the caller thread
+    void DoneCurrent() override;
+
+    /// Whether the window is still open, and a close request hasn't yet been sent
+    bool IsOpen() const;
+
+private:
+    /// Whether the GPU and driver supports the OpenGL extension required
+    bool SupportsRequiredGLExtensions();
+
+    /// Internal SDL2 render window
+    SDL_Window* render_window;
+
+    using SDL_GLContext = void*;
+    /// The OpenGL context associated with the window
+    SDL_GLContext gl_context;
+};