Blender Git Loki

Bug fix #33639

Material Texture blend modes Screen, Overlay and Multiply didn't respect
the alpha for textures... an error I could trace back to 2004 even.

Obviously the fix should be done, but it might change the appearance of
renders somewhat. Will keep an eye open if this is worth ugly
version-patching.

Now: image textures with alpha, will only apply the blend modes
respecting the alpha values.