Multiple materials to single texture?

Are there any applications out there that "merge" the (multiple) textures of a single mesh into a single texture with matching uv? It's a lot easier for me to model, say, a tree with separate materials for the leaves and for the bark, but since there's no other good reason to have the materials separate (plus it hurts performance), I wish there was an easy way to just merge the parts.

I use C4D to model and get the feeling the option is in there somewhere, but it's such a complicated app that it could easily be under my nose. Or maybe there are others?