VBO revisit

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see http://www.learnopengles.com/tag/vertex-buffer-object/
and https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/TechniquesforWorkingwithVertexData/TechniquesforWorkingwithVertexData.html

http://www.learnopengles.com/android-lesson-seven-an-introduction-to-vertex-buffer-objects-vbos/

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I'm implementing VBO.

(ref) ... Each time DrawModel is called, the index and vertex data are copied to OpenGL ES, and transferred to the graphics hardware. If the vertex data does not change between invocations, these unnecessary copies can impact performance. To avoid unnecessary copies, your app should store its vertex data in a vertex buffer object (VBO). Because OpenGL ES owns the vertex buffer object’s memory, it can store the buffer in memory that is more accessible to the graphics hardware, or pre-process the data into the preferred format for the graphics hardware.

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TODO: check this again 09d96a4