model transparency / opacity

[Ithamar]

Currently I create the model textures with (1-bit) alpha based on color (black is transparent). For this to properly work, we need to create the geometry in two groups, one is transparent faces, one is not.
Then we can pass 2 materials, one transparent, and one not, and that should get rid of the transparency issues.

If this is no option, we have to look into a custom shader for models as well.