Implement draw call batching for repeat draw calls

[nsgomez]

This is an extension of issue #8. SCGL directly calls glDrawElements for every cGDriver::DrawElements call, which intuitively makes sense, but also causes a lot of overhead when there are several draw calls in a row being made for the same object, just in different places.

The game's DirectX driver has a PrimitiveManager that sits between the game's graphics director and the graphics API, which allows it to batch draw calls together when the primitives being drawn are all the same. Something similar for SCGL should help improve performance.