Plane clipping

Desperately needed clipping for my glsl-water, so I hacked it in as a new renderstate…fast hack so I dunno if it's the best way, but at least it works…

maybe renanse or mojo can use some of it…

Added files:

 * Copyright (c) 2003-2005 jMonkeyEngine
 * All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
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Forgot, this needs to be added to the end of updateStates in as well

      Spatial.defaultStateList[RenderState.RS_CLIP] = r