Diffuse Alpha support for Material texture

My apologies for the mess. I'm still not used to GitHub. I mistakenly deleted my repository with the result previous commits can't be merged.

I hope there's a way to remove the previous pull request, Goran. Anyhow, just repeating:

See Issue #42: Material alpha overwrites texture alpha (v0.0.69).
Note: changes in texture_name() are removal of unnecessary check whether there is a material, plus addition of a check whether the texture slot is active.