Support SSAO

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Screen-Space Ambient Occlusion

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http://codeflow.org/webgl/ssao/

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http://blog.csdn.net/xoyojank/article/details/5734537

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Occlusion = max( 0.0, dot( N, V) ) * ( 1.0 / ( 1.0 + d ) )

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https://zhuanlan.zhihu.com/p/25038820

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https://learnopengl-cn.github.io/05%20Advanced%20Lighting/09%20SSAO/

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Normal-oriented Hemisphere

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• https://github.com/pyalot/WebGL-City-SSAO
  http://codeflow.org/webgl/ssao/

• https://github.com/shama/voxel-ssao
  http://shama.github.io/voxel-ssao/