Clarify Bug 692: Unbinding a VAO does _not_ imply unbinding of set VBOs (spec doesn't mention it, and it does not show results w/ CPU sourced rendering) ; Clean up GLBuffer*Tracker

+ * Note that VAO initialization does unbind the VBO .. since otherwise they are still bound
+ * and the CPU_SRC test will fail!<br/>
+ * The OpenGL spec does not mention that unbinding a VAO will also unbind the bound VBOs
+ * during their setup.<br/>
+ * Local tests here on NV and AMD proprietary driver resulted in <i>no ourput image</i>
+ * when not unbinding said VBOs before the CPU_SRC tests.<br/>
+ * Hence Bug 692 Comment 5 is invalid, i.e. <https://jogamp.org/bugzilla/show_bug.cgi?id=692#c5>,
+ * and we should throw an exception to give users a hint!

Leaving uncommented code in GLBufferStateTracker ..

+++

- Clean up GLBuffer*Tracker
  - Use final
  - Use static final keyNotFound value.