Support for NiTriShape particle emitters

NIF particle systems can set their emitter to a NiTriShape, in which case particles are spawned on the vertices defined in the geometry data. Currently, OpenMW does not seem to support this.

This is used in a few mods, notably with a NiBSPArrayController with a NiTriShape emitter. OAAB increasingly has made use of this feature.

I have no yet confirmed what happens when the emitter is a node with children. At first glance it seems only the first NiTriShape's geometry is being used, but this needs verifying.