beginWithClear() {
    if (!this._redirectingToScreenPass) {
      // render all instances
      this._renderForInstancing();

      // redirect all drawing into screen pass
      // but only if we'll actually use it
      if (this.getIsBlurringScreen() || this.getScreenCacheRequested()) {
        if (!this.getIsCachingScreen()) {
          this.passes.screen.beginWithResetClear(this._postProcessingRenderPassStackId);
          this._redirectingToScreenPass = true;
        }
      }
    }
  }
Begins redirecting drawing into screen framebuffers.
NOTE: This should be called automatically at the beginning of the frame rendering phase.