Added support for custom system cursors.

This adds 3 functions to the GLFW API: glfwCreateCursor,
glfwDestroyCursor and glfwSetCursor.

src/init.c

@@ -156,6 +156,10 @@ GLFWAPI void glfwTerminate(void)
     while (_glfw.windowListHead)
         glfwDestroyWindow((GLFWwindow*) _glfw.windowListHead);
 
+    // Destroy all cursors
+    while (_glfw.cursorListHead)
+        glfwDestroyCursor((GLFWcursor*) _glfw.cursorListHead);
+
     for (i = 0;  i < _glfw.monitorCount;  i++)
     {
         _GLFWmonitor* monitor = _glfw.monitors[i];