Function chooseSharedRtcOffset

The sharedRtcOffset a newly-loading model should be given: the earliest already-loaded model's wasmRtcOffset, or undefined when no already-loaded model has one yet (including when there is no earlier model at all) — the loading model is then free to detect its own.

The same rule picks the anchor every already-loaded model is converged onto after a load (rtc-rebase.ts), so a model loaded before the first anchored one is moved onto it rather than left in the raw frame it was meshed in.