Register GetMapProducers directly and remove the factories

For the moment we do have GetMapProducerSpi and GetMapProducer, and we do register the factories onto the context.
I suggest to remove the factories and register directly the GetMapProducer, since it's simpler.

For producers that are not thread safe, just register them as prototypes.
For producers that do need external configuration to decide on a code path (like svg), just add a delegate producers that performs the decision.