RGL object fetching reworked

[pijaro]

The approach for the system looks as follows:

• Fetch all the scene objects on the initialization, only once,
• Each object spawned on the scene (vehicle or pedestrian) registers itself in the RGL SceneManager.
• Each object despawned from the scene (vehicle or pedestrian) unregisters itself in from the RGL SceneManager.

In a result, there are no frequent fetches of the whole level (FindObjectsOfType).

Requires #148

[mackierx111]

Change to draft until #148 is merged

[ceccocats]

Nice work :+1: Unfortunately SceneManager: Update transforms and skinned meshes is still slow. It would be possible to just update objects that are not marked static in unity?