[GPU] Improvements to GPU settings modification

- Renamed CommonGPUSetting to GPUSetting
- Removed readback_resolve and memexport from d3d12 exclusive option. In the future it will be available for Vulkan too.
- Removed unused enum class: gpu_cvar
- Removed OS specific code from emulator_window

src/xenia/gpu/command_processor.h

@@ -33,11 +33,14 @@ class ByteStream;
 
 namespace gpu {
 
-enum class CommonGPUSetting {
+enum class GPUSetting {
   ClearMemoryPageState,
+  ReadbackResolve,
+  ReadbackMemexport
 };
 
-void CommonSaveGPUSetting(CommonGPUSetting setting, uint64_t value);
+void SaveGPUSetting(GPUSetting setting, uint64_t value);
+bool GetGPUSetting(GPUSetting setting);
 
 class GraphicsSystem;
 class Shader;