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.cc

@@ -0,0 +1,53 @@
+/**
+ ******************************************************************************
+ * 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. *
+ ******************************************************************************
+ */
+
+#include "xenia/gpu/null/null_graphics_system.h"
+
+#include "xenia/gpu/null//null_command_processor.h"
+#include "xenia/ui/vulkan/vulkan_provider.h"
+#include "xenia/xbox.h"
+
+namespace xe {
+namespace gpu {
+namespace null {
+
+NullGraphicsSystem::NullGraphicsSystem() {}
+
+NullGraphicsSystem::~NullGraphicsSystem() {}
+
+X_STATUS NullGraphicsSystem::Setup(cpu::Processor* processor,
+                                   kernel::KernelState* kernel_state,
+                                   ui::Window* target_window) {
+  // This is a null graphics system, but we still setup vulkan because UI needs
+  // it through us :|
+  provider_ = xe::ui::vulkan::VulkanProvider::Create(target_window);
+
+  return GraphicsSystem::Setup(processor, kernel_state, target_window);
+}
+
+void NullGraphicsSystem::Shutdown() { GraphicsSystem::Shutdown(); }
+
+std::unique_ptr<CommandProcessor> NullGraphicsSystem::CreateCommandProcessor() {
+  return std::unique_ptr<CommandProcessor>(
+      new NullCommandProcessor(this, kernel_state_));
+}
+
+void NullGraphicsSystem::Swap(xe::ui::UIEvent* e) {
+  if (!command_processor_) {
+    return;
+  }
+
+  auto& swap_state = command_processor_->swap_state();
+  std::lock_guard<std::mutex> lock(swap_state.mutex);
+  swap_state.pending = false;
+}
+
+}  // namespace null
+}  // namespace gpu
+}  // namespace xe