Copy triangle data from NiSkinPartition, a.k.a. SKYRIM TREES

I have suffered for you to see TES5 support, and I see how you love Skyrim trees. Let me show you the power of Capo Nifcrown, born of the North, where my breath is long winter. I breathe now, in royalty, and reshape this land which is yours. I do this for you, OpenMW Legions, for I love you.

NiSkinPartition triangle data is the source geometry for skinning which replaces the triangles from NiGeometry. This specific implementation is kind of ugly, but we'd eventually have to do something like this anyway. Also I really want Skyrim trees.

Most notably allows the main (skinned) version of Skyrim trees to show up without hacking NiSwitchNodes.

May need to be updated for SSE in the future. ptmikheev was concerned that their use everywhere might have a performance impact with their animations active. I think we shouldn't try to optimize this prematurely, though.