Added glfwIsWindow.

include/GL/glfw.h

diff --git a/include/GL/glfw.h b/include/GL/glfw.h
index 1f2bb8a..4647b7b 100644
--- a/include/GL/glfw.h
+++ b/include/GL/glfw.h
@@ -391,6 +391,7 @@
 GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int redbits, int greenbits, int bluebits, int alphabits, int depthbits, int stencilbits, int mode);
 GLFWAPI void glfwOpenWindowHint(int target, int hint);
 GLFWAPI void glfwMakeWindowCurrent(GLFWwindow window);
+GLFWAPI int  glfwIsWindow(GLFWwindow window);
 GLFWAPI void glfwCloseWindow(GLFWwindow window);
 GLFWAPI void glfwSetWindowTitle(GLFWwindow, const char* title);
 GLFWAPI void glfwGetWindowSize(GLFWwindow, int* width, int* height);