Playback rewind has edge cases

[brentyi]

Incremental updates to scene node properties (visibility, position, point cloud size, etc) aren't correctly reset when we rewind:

viser/src/viser/client/src/FilePlayback.tsx

Lines 170 to 173 in 440a3e2

	if (value < playbackMutable.current.currentTime) {
	// Going backwards is more expensive...
	playbackMutable.current.currentIndex = recording!.loopStartIndex!;
	}

The easiest fix is to set playbackMutable.current.currentIndex to 0 instead of the loop start index, but this has performance implications. Needs some thought.