onDoubleTap method
Implementation
void onDoubleTap(double focalX, double canvasWidth) {
if (_disposed) return;
if (!constraints.enableDoubleTapZoom) return;
if (canvasWidth <= 0 || !canvasWidth.isFinite || !focalX.isFinite) return;
final frac = value.canvasToFraction(focalX, canvasWidth);
value = value.zoomAroundFraction(frac, constraints.doubleTapZoomFactor);
_applyConstraints();
}