Generation stops if user leaves page while in progress

[takempf]

From @brandoelettroliti-a11y in another issue (#88)

Also, if I leave the page and go to another one, or if I touch the page while the GIF creation process is running, the process stops (Brave/Vivaldi).

[takempf]

Unfortunately, the popup has to remain open and on the same page for the GIF to complete generation. The actual <video> element on the page must remain visible the entire time, and the popup UI needs to be present to copy images.