SDL2: set relative mouse mode when grabbing the mouse

Reported-by: heftig <https://github.com/dxx-rebirth/dxx-rebirth/issues/397>

common/arch/sdl/event.cpp

@@ -201,6 +201,7 @@ static void event_change_focus()
 	SDL_WM_GrabInput(enable_grab ? SDL_GRAB_ON : SDL_GRAB_OFF);
 #elif SDL_MAJOR_VERSION == 2
 	SDL_SetWindowGrab(g_pRebirthSDLMainWindow, enable_grab ? SDL_TRUE : SDL_FALSE);
+	SDL_SetRelativeMouseMode(enable_grab ? SDL_TRUE : SDL_FALSE);
 #endif
 	if (activate_focus)
 		mouse_disable_cursor();