Reduce vertexbuffer allocations

[kennyvv]

Currently we have a lot of vertexbuffers for all sorts of different things. This causes a lot of lag when spawning or loading & updating chunks. All of these operations need to be executed on the UI/Rendering thread. To fight this we should reduce the amount of allocations & updates.

For example:

• Chunks have multiple vertexbuffers, 1 for each renderstage required. (Renderstages seperate different block types like translucent, animated or solid blocks)
• For each entity a new vertexbuffer is created for it's model, model instancing should be used to reduce this as lots of entities share the same basic model.

[kennyvv]

We could also start using indexbuffers for chunks again, this way when a block is removed from a chunk all we have to do is remove the indexes to that block's data from the indexbuffer instead of rebuilding the whole vertexbuffer.

[kennyvv]

Closing this issue, a better more detailed version is available in issue #106