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/premake5.lua

@@ -0,0 +1,21 @@
+project_root = "../../../.."
+include(project_root.."/tools/build")
+
+group("src")
+project("xenia-gpu-null")
+  uuid("42FCA0B3-4C20-4532-95E9-07D297013BE4")
+  kind("StaticLib")
+  language("C++")
+  links({
+    "xenia-base",
+    "xenia-gpu",
+    "xenia-ui",
+    "xenia-ui-vulkan",
+    "xxhash",
+  })
+  defines({
+  })
+  includedirs({
+    project_root.."/third_party/gflags/src",
+  })
+  local_platform_files()