iOS/safari canvas memory leak

[cyptus]

signaturepad leads into an memory leak in iOS because the canvas is not destroyed correctectly: https://stackoverflow.com/questions/52532614/total-canvas-memory-use-exceeds-the-maximum-limit-safari-12

The error occurred under iOS after multiple instances have been created over the lifetime of an app. It seems to be an Bug in WebKit by Apple which wont get fixed from angular.

I fixed this by implementating OnDestroy and setting canvas width and height to zero, like mentioned in the stackoverflow post:

  ngOnDestroy() {
    this.signaturePad.set('canvasHeight', 0);
    this.signaturePad.set('canvasWidth', 0);
  }

SignaturePad Component should already do this itself as this error is hard to find.