inline models don't take model slots

[BraXi]

The changes to inline models:

• inline models are not written to config strings, thus are not sent to client, the client knows all bmodels anyway since they're stored in BSP
• inline models no longer take svmodel_s slots, thus they're no longer counting towards MAX_MODELS limit
• all inline models (except the BSP world model) are accessed via negative model indexes
• the net protocol does send models as short, allowing for 65k models in total, since modelindexes can be negative sending them as char/byte is a bit too limiting
• a bit more strict renderer api check that verifies shared struct sizes