Input: Remove global variables from SDL Input

Changes the interface as well to remove any unique methods that
frontends needed to call such as StartJoystickEventHandler by
conditionally starting the polling thread only if the frontend hasn't
started it already. Additionally, moves all global state into a single
SDLState class in order to guarantee that the destructors are called in
the proper order

src/input_common/sdl/sdl.h

@@ -1,4 +1,4 @@
-// Copyright 2017 Citra Emulator Project
+// Copyright 2018 Citra Emulator Project
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
@@ -7,45 +7,36 @@
 #include <memory>
 #include <vector>
 #include "core/frontend/input.h"
+#include "input_common/main.h"
 
 union SDL_Event;
+
 namespace Common {
 class ParamPackage;
-}
-namespace InputCommon {
-namespace Polling {
+} // namespace Common
+
+namespace InputCommon::Polling {
 class DevicePoller;
 enum class DeviceType;
-} // namespace Polling
-} // namespace InputCommon
+} // namespace InputCommon::Polling
 
-namespace InputCommon {
-namespace SDL {
+namespace InputCommon::SDL {
 
-/// Initializes and registers SDL device factories
-void Init();
+class State {
+public:
+    /// Unresisters SDL device factories and shut them down.
+    virtual ~State() = default;
 
-/// Unresisters SDL device factories and shut them down.
-void Shutdown();
+    virtual std::vector<std::unique_ptr<InputCommon::Polling::DevicePoller>> GetPollers(
+        InputCommon::Polling::DeviceType type) = 0;
+};
 
-/// Needs to be called before SDL_QuitSubSystem.
-void CloseSDLJoysticks();
+class NullState : public State {
+public:
+    std::vector<std::unique_ptr<InputCommon::Polling::DevicePoller>> GetPollers(
+        InputCommon::Polling::DeviceType type) override {}
+};
 
-/// Handle SDL_Events for joysticks from SDL_PollEvent
-void HandleGameControllerEvent(const SDL_Event& event);
+std::unique_ptr<State> Init();
 
-/// A Loop that calls HandleGameControllerEvent until Shutdown is called
-void PollLoop();
-
-/// Creates a ParamPackage from an SDL_Event that can directly be used to create a ButtonDevice
-Common::ParamPackage SDLEventToButtonParamPackage(const SDL_Event& event);
-
-namespace Polling {
-
-/// Get all DevicePoller that use the SDL backend for a specific device type
-std::vector<std::unique_ptr<InputCommon::Polling::DevicePoller>> GetPollers(
-    InputCommon::Polling::DeviceType type);
-
-} // namespace Polling
-} // namespace SDL
-} // namespace InputCommon
+} // namespace InputCommon::SDL