jsPsychSketchpad background image cross origin issue

[Shaobin-Jiang]

See discussion originally posted by @HenningTm in https://github.com/jspsych/jsPsych/discussions/3224.

Setting save_final_image property of jsPsychSketchpad plugin to true, which invokes the toDataURL method of a canvas, causes Tainted canvases may not be exported error. This might be fixed by adding this.background_image.crossOrigin = 'Anonymous'; during image loading.