[native] Add SDL2 native headers

Not included by DXVK at all, but used by external applications.

include/native/wsi/native_sdl2.h

@@ -0,0 +1,25 @@
+#include <windows.h>
+
+#include <SDL2/SDL.h>
+
+namespace dxvk::wsi {
+
+  inline SDL_Window* fromHwnd(HWND hWindow) {
+    return reinterpret_cast<SDL_Window*>(hWindow);
+  }
+
+  inline HWND toHwnd(SDL_Window* pWindow) {
+    return reinterpret_cast<HWND>(pWindow);
+  }
+
+  // Offset so null HMONITORs go to -1
+  inline int32_t fromHmonitor(HMONITOR hMonitor) {
+    return static_cast<int32_t>(reinterpret_cast<intptr_t>(hMonitor)) - 1;
+  }
+
+  // Offset so -1 display id goes to 0 == NULL
+  inline HMONITOR toHmonitor(int32_t displayId) {
+    return reinterpret_cast<HMONITOR>(static_cast<intptr_t>(displayId + 1));
+  }
+
+}

include/native/wsi/native_wsi.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#ifdef DXVK_WSI_WIN32
+#error You shouldnt be using this code path.
+#elif DXVK_WSI_SDL2
+#include "wsi/native_sdl2.h"
+#else
+#error Unknown wsi!
+#endif