Unity透明地形

http://answers.unity3d.com/questions/1162779/unity-5-transparent-terrain-shader.html

http://answers.unity3d.com/questions/1077032/terrain-transparency-too-many-text-interpolators.html

http://wiki.unity3d.com/index.php/TerrainTransparency

https://alastaira.wordpress.com/2013/12/07/custom-unity-terrain-material-shaders/

https://docs.unity3d.com/ScriptReference/TerrainData.GetAlphamaps.html

https://docs.unity3d.com/ScriptReference/Texture2D.EncodeToPNG.html