Class BatchModelRenderer: Model renderer that sorts submitted models
by CModelDef and texture for batching, and uses a ModelVertexRenderer
(e.g. FixedFunctionModelRenderer) to manage model vertices.
Struct BatchModelRendererInternals: Internal data of the BatchModelRenderer
Separated into the source file to increase implementation hiding (and to
avoid some causes of recompiles).
allocator design goals:
- either fixed- or variable-sized blocks;
- allow freeing individual blocks if they are all fixed-size;
- never relocates;
- no fixed limit.