Moved GL_ALPHA_TEST from AlphaFunc header to GLDefines header

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@15002 16af8721-9629-0410-8352-f15c8da7e697

include/osg/GLDefines

@@ -600,6 +600,10 @@ typedef char GLchar;
 #define GL_RASTERIZER_DISCARD           0x8C89
 #endif
 
+#ifndef GL_ALPHA_TEST
+    #define GL_ALPHA_TEST 0x0BC0
+#endif
+
 namespace osg
 {
     #ifndef GL_VERSION_3_2