Added window position callback.

diff --git a/src/internal.h b/src/internal.h
index c5a40af..2ef8f10 100644
--- a/src/internal.h
+++ b/src/internal.h

@@ -199,6 +199,7 @@
     int       glRobustness;
     PFNGLGETSTRINGIPROC GetStringi;
 
+    GLFWwindowposfun     windowPosCallback;
     GLFWwindowsizefun    windowSizeCallback;
     GLFWwindowclosefun   windowCloseCallback;
     GLFWwindowrefreshfun windowRefreshCallback;