New extract_texture(...) is missing alpha-channel weighting

[patrikhuber]

In the devel branch, in 08cba28, I replaced the old extract_texture(...) with the newer v2::extract_texture(...), and made the new one the default by removing the v2:: from the namespace. See the commit messages of 08cba28 and 8fd1735 for more information.

The thing to note here is that the new function does not (yet) support computing and outputting the view-angle based weighting, which was previously output in the alpha channel. This has to be re-added. To be consistent with the old implementation, the visibility in the 4th channel was 0=invis, 255=visible.