fix: panel flashes when using virtual option in react

[malangfox]

Issue

#915

Details

After the display: none style is applied to hidden Panel, there appears to be a flash before the correct transform is applied.

applyTransform does not work when renderer.rendering is active.

In the ReactRenderer, I modified the execution order as follows so that applyTransform is applied together with Panel addition/removal:

  public async render() {
    const flicking = getFlickingAttached(this._flicking);
    const reactFlicking = this._reactFlicking;
    const strategy = this._strategy;

    strategy.updateRenderingPanels(flicking);
    this._rendering = true;

    strategy.renderPanels(flicking);
  }

The key change is that this._rendering = true; has been moved below updateRenderingPanels.

By implementing the modification above, the issue is resolved without needing to call applyTransform in beforeRender.

• renderPanels is responsible for adding or removing Panels in the virtual area.
• updateRenderingPanels calls camera.updateOffset();.
• Since updateOffset calls applyTransform, delaying the timing of setting renderer.rendering to true ensures that the translation values are correctly applied in accordance with the state where Panels are added/removed in renderPanels.

[coveralls]

coverage: 82.673%. remained the same
when pulling fa073c0 on malangfox:fix/react-virtual
into 293da58 on naver:master.