This unifies the various factory-options, since it's consistent with `CMapReaderFactory`/`StandardFontDataFactory`, and ensures that any needed parameters will always be consistently provided when creating `CanvasFactory`/`FilterFactory`-instances.
As shown in the modified example this may simplify some custom implementations, since we now provide the ability to access the `CanvasFactory`-instance used with a particular `getDocument`-invocation.
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