Refactor|RenderSystem: Write to the map vertex buffer independently of DrawList

Rather than pass geometry data into DrawList to perform the vertex
buffer write, perform the write first and then provide the vertex
indices to DrawList::write()

As of this commit it is now possible to write to RenderSystems's
world-wide vertex buffer without the need for extra working buffers
(beside the indices, that is).

Todo:
- Update all generation to use the world-wide vertex buffer directly
- Cleanup