syncViewport method
Implementation
void syncViewport(ZoomRange zoom, double panOffset) {
if (_disposed) return;
final nextZoom = zoom.normalized();
final nextPanOffset = _finiteOr(panOffset, 0);
if (_zoom == nextZoom && _panOffset == nextPanOffset) return;
_zoom = nextZoom;
_panOffset = nextPanOffset;
_notifyIfAlive();
}