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_command_processor.cc

@@ -0,0 +1,50 @@
+/**
+ ******************************************************************************
+ * 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_command_processor.h"
+
+namespace xe {
+namespace gpu {
+namespace null {
+
+NullCommandProcessor::NullCommandProcessor(NullGraphicsSystem* graphics_system,
+                                           kernel::KernelState* kernel_state)
+    : CommandProcessor(graphics_system, kernel_state) {}
+NullCommandProcessor::~NullCommandProcessor() = default;
+
+bool NullCommandProcessor::SetupContext() {
+  return CommandProcessor::SetupContext();
+}
+
+void NullCommandProcessor::ShutdownContext() {
+  return CommandProcessor::ShutdownContext();
+}
+
+void NullCommandProcessor::PerformSwap(uint32_t frontbuffer_ptr,
+                                       uint32_t frontbuffer_width,
+                                       uint32_t frontbuffer_height) {}
+
+Shader* NullCommandProcessor::LoadShader(ShaderType shader_type,
+                                         uint32_t guest_address,
+                                         const uint32_t* host_address,
+                                         uint32_t dword_count) {
+  return nullptr;
+}
+
+bool NullCommandProcessor::IssueDraw(PrimitiveType prim_type,
+                                     uint32_t index_count,
+                                     IndexBufferInfo* index_buffer_info) {
+  return true;
+}
+
+bool NullCommandProcessor::IssueCopy() { return true; }
+
+}  // namespace null
+}  // namespace gpu
+}  // namespace xe