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.
Oops, you're right. Sorry, my C++ is apparently a bit rusty, I totally forgot that caveat about `resize()`; I got lulled into committing that because the regression tests passed on my end (so much for "fail in practice", heh), but of course relying on undefined behaviour (`operator[]` beyond visible size) is never a good idea.
How about I throw that local copy out altogether, and just do the lookups into the passed reference to `indices` directly. That way, we avoid allocations per-vertex, `indices[x]` should still be O(1), and `CopyVertex` does most of the math required already anyway.