Add a null graphics system (enable with --gpu=null)

Still uses vulkan to drive xenia UI, may look into decoupling later.

src/xenia/gpu/null/null_graphics_system.h

@@ -0,0 +1,41 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2016 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_GPU_NULL_NULL_GRAPHICS_SYSTEM_H_
+#define XENIA_GPU_NULL_NULL_GRAPHICS_SYSTEM_H_
+
+#include <memory>
+
+#include "xenia/gpu/graphics_system.h"
+#include "xenia/gpu/command_processor.h"
+
+namespace xe {
+namespace gpu {
+namespace null {
+
+class NullGraphicsSystem : public GraphicsSystem {
+ public:
+  NullGraphicsSystem();
+  ~NullGraphicsSystem() override;
+
+  X_STATUS Setup(cpu::Processor* processor, kernel::KernelState* kernel_state,
+                 ui::Window* target_window) override;
+  void Shutdown() override;
+
+ private:
+  std::unique_ptr<CommandProcessor> CreateCommandProcessor() override;
+
+  void Swap(xe::ui::UIEvent* e) override;
+};
+
+}  // namespace null
+}  // namespace gpu
+}  // namespace xe
+
+#endif  // XENIA_GPU_NULL_NULL_GRAPHICS_SYSTEM_H_