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DO NOT use template programming in VertexBuffer

Still have issue in breaking down code to Bindable/Drawable, while in Triangle it fails in link stage.
Only able to init3D and render single color in page.

viewport.MinDepth and viewport.MaxDepth were not initialized, so they were both 0, which prevents the depth buffer from being calculated correctly.
fix this issue by explicitly specify viewport.MinDepth as 0 and viewport.MaxDepth as 1.