Record entity count in a log file.

[nkoenig]

We need an entity ID offset, stored in a log file, in order to support spawning of new objects during log playback.

Logplayback will directly insert entities with their recorded ids, which doesn't alter the entityCount. As a result, a spawn call will attempt to create an entity with an existing ID.

A work-around is in #346 here.

[chapulina]

support spawning of new objects during log playback.

What's the use case? Is this for visualization purposes? Typically, during log playback, physics and other systems are not running.

[nkoenig]

The initial use case is to add lights during log playback.

[nkoenig]

We will likely also want to spawn models with camera sensors.