Minor tweaks to material.

* Use 100% white transparency color when no transparency at all (seems to be needed by some importers).
* Always export some kind of Lambert shader, even when using e.g. Blender's toon one (better to get something, even inacurate, than nothing).

Patch by Jens Restemeier, many thanks!

Differential Revision: https://developer.blender.org/D405