Fixed issue with Struct Batching

Batching Plugin + Structs

As the issue with structs and batches still seemed to have no real avenue to being resolved other than only allowing blittable types, the decision has been made to just make this a requirement.

If you do not use the batching plugin then your structs (if you use them) can be of any type you want, but if you want to get the performance boost of them all being batched together you need to use blittable types otherwise they cannot be pinned.

Also updated group builder

Added a couple of struct methods to the group builder, this should allow you to use the builder to make groups with struct types in too