Renderer: Finalized geometry generation changes (cleanup)

As of this commit, all drawable map geometry is now written directly
to the world-wide vertex buffer, using only RenderSystem's pooled,
vertex indice buffers.

Vertex buffer writes now occur before the primitives are added to the
relevant DrawList(s). Meaning, the two can now be separated in future.

(In the next phase, vertex buffer writes could be encapsulated inside
SectorCluster, storing buffer indices and draw list specs in Shards).