Add DepthTexture for PostProcessing

[GiveOneMoreChance]

It give more wider features for PostProcessing e.g DoF, cell shading, fog

[dpJudas]

At the moment the depth texture is partially destroyed due to the way the portals are implemented. If the current behavior is exported to mods it will lock the implementation in stone and we will never be able to use other ways. For that reason it is too risky to give access to the depth texture in postprocessing shaders.

If you want DoF you'll have to do it as a pull request.