Changes

http://www.jogamp.org/bugzilla/show_bug.cgi?id=391

As shown below, the concurrent access of GLStateTracker
while being cleared by GLContext.destroy() causes a NPE.

GLContext.destroy() shall disable it first, then clear it.

The state stack pop method shall also swap the mapping
with a most atomic action.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.sun.opengl.impl.GLStateTracker.getInt(GLStateTracker.java:94)
        at com.sun.opengl.impl.gl2.GL2Impl.glGetIntegerv(GL2Impl.java:6102)
        at com.sun.opengl.impl.gl2.GL2Impl.imageSizeInBytes(GL2Impl.java:26000)
        at com.sun.opengl.impl.gl2.GL2Impl.imageSizeInBytes(GL2Impl.java:25713)
        at com.sun.opengl.impl.gl2.GL2Impl.glTexImage2D(GL2Impl.java:18692)
        at glredbook1314.combiner.init(combiner.java:104)
        at com.sun.opengl.impl.GLDrawableHelper.init(GLDrawableHelper.java:88)
        at javax.media.opengl.awt.GLJPanel$Updater.init(GLJPanel.java:557)
        at com.sun.opengl.impl.GLDrawableHelper.init(GLDrawableHelper.java:88)
        at
com.sun.opengl.impl.GLPbufferImpl$InitAction.run(GLPbufferImpl.java:274)
        at
com.sun.opengl.impl.GLDrawableHelper.invokeGL(GLDrawableHelper.java:149)
        at
com.sun.opengl.impl.GLPbufferImpl.maybeDoSingleThreadedWorkaround(GLPbufferImpl.java:267)
        at
com.sun.opengl.impl.GLPbufferImpl.swapBuffers(GLPbufferImpl.java:157)
        at
com.sun.opengl.impl.GLDrawableHelper.invokeGL(GLDrawableHelper.java:159)
        at javax.media.opengl.awt.GLJPanel.dispose(GLJPanel.java:238)

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