Make "normalized sess. jitter" accurate at the beginning of the session.

[Dan-Linden]

Environment

Second Life Release 7.2.2.18475198968 (64bit)

Description

While gathering performance benchmarks for the Native Apple Silicon Client, some patterns were noticed.

Immediately after logging in the "normalized sess. jitter" is often much higher than "normalized period jitter". It can take 5 to 10 minutes for "normalized sess. jitter" to match "normalized period jitter". This decreases the value of "normalized sess. jitter" for performance testing.

There is probably a lot of jitter when the viewer rendering initializes (or before the rendering initializes?), but I don't think this is valuable data.
I propose that the "normalized sess. jitter" value should start accumulating a second or two after the world starts rendering for the first time, so that the "normalized sess. jitter" is accurate from the beginning of the session.

Reproduction steps

1. Log in
2. Open Statistics (ctrl+shift+1)
3. Check how close "normalized sess. jitter" is to "normalized period jitter"

Expected: They should be very close at the beginning of the session.

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