Collada: tristrip support

We needed to import some COLLADA files created from a CAD exporter (which shall remain unnamed, in order to protect the innocent) which contained tristrips. As we had no way to dissuade the exporter from writing tristrips, we decided to implement tristrips in assimp's COLLADA parser.

The commits in this pull request were created as a pair programming effort, so I left myself in as Committer and my colleague as Author. If you have any better Ideas for correct attribution let me know, I'll gladly amend.
Hmm, this method is called per vertex so I'm not sure that we want to heap allocate for every vertex. Ideally we'd use some kind of stack allocated vector (not sure if we support that? if not we should definitely consider adding a stack allocator). The alternative would be creating the vector once, and passing it into each function call.

Also, reserve doesn't actually change the visible size of the vector, so the assignments below will fail in practice.