Introduce dummy context

src/yuzu_cmd/emu_window/emu_window_sdl2_vk.cpp

@@ -71,7 +71,7 @@ EmuWindow_SDL2_VK::EmuWindow_SDL2_VK(Core::System& system, bool fullscreen)
 EmuWindow_SDL2_VK::~EmuWindow_SDL2_VK() = default;
 
 std::unique_ptr<Core::Frontend::GraphicsContext> EmuWindow_SDL2_VK::CreateSharedContext() const {
-    return nullptr;
+    return std::make_unique<DummyContext>();
 }
 
 void EmuWindow_SDL2_VK::Present() {

src/yuzu_cmd/emu_window/emu_window_sdl2_vk.h

@@ -22,3 +22,5 @@ public:
 
     std::unique_ptr<Core::Frontend::GraphicsContext> CreateSharedContext() const override;
 };
+
+class DummyContext : public Core::Frontend::GraphicsContext {};