Make it possible to generate srcset with the thumbnail template tag

[autoantwort]

Instead of

{% thumbnail 'x400' image.image as img %}
{% thumbnail 'x800' image.image as img2 %}
{% thumbnail 'x1200' image.image as img3 %}
    <img src="{{ img.url }}"
         srcset="{{ img.url }}, {{ img2.url }} 2x, {{ img3.url }} 3x"
         height="{{ img.height }}" width="{{ img.width }}">

one can write

{% thumbnail 'x400' bar.image srcset="2 3" %}

[vstoykov]

Thank you @autoantwort for your contribution. Can I ask you to add simple test that is rendering the template tag and observe the output of it?