Change SDL to not use DefWindowProc as the window class' window procedure.

For some reason, having lpfnWndProc=DefWindowProc during window class registration causes IME input to not work in windows.
With this small change, IME input should now work in SDL windows but not in fullscreen mode.

src/video/win32/SDL_win32events.c

@@ -703,7 +703,7 @@ SDL_RegisterApp(char *name, Uint32 style, void *hInst)
     class.hbrBackground = NULL;
     class.hInstance = SDL_Instance;
     class.style = SDL_Appstyle;
-    class.lpfnWndProc = DefWindowProc;
+    class.lpfnWndProc = WIN_WindowProc;
     class.cbWndExtra = 0;
     class.cbClsExtra = 0;
     if (!RegisterClass(&class)) {

src/video/win32/SDL_win32window.c

@@ -112,7 +112,7 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, SDL_bool created)
 
     /* Set up the window proc function */
     data->wndproc = (WNDPROC) GetWindowLongPtr(hwnd, GWLP_WNDPROC);
-    if (data->wndproc == DefWindowProc) {
+    if (data->wndproc == WIN_WindowProc) {
         data->wndproc = NULL;
     }
     SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR) WIN_WindowProc);