Now that we've extracted all of the stateless functions that can be
hidden, it's time to make the index generator a regular class with
active data members.
This can just be a member that sits within the vertex manager base
class. By deglobalizing the state of the index generator we also get rid
of the wonky dual-initializing that was going on within the OpenGL
backend.
Since the renderer is always initialized before the vertex manager, we
now only call Init() once throughout the execution lifecycle.
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