Added support for instancing at the mesh level.
(so far only for lwjgl because I'm lazy)
Went ahead and added in divisor support while I was
at it because it doesn't cost anything and there might
be use-cases for it.
Mesh now knows about instances and you can directly
ask it for an instance count.  I haven't added support
for proper bounding box calculation but it should
at least be possible now.