Do not regenerate the webp file each time it is requested

Rerolling #21 to work with RC1 release.

Rerolling #22 to work with RC1 release.

This issue is related to 🐛 Webp derivatives not refreshed when files stored in sub-directory Needs review . When that other issue is merged, the code in the new getWebpCopy() method provided by this patch will need to be updated to
$destination = $this->getWebpDestination($uri, '@directory/@filename.webp');

Can someone provide steps to reproduce? Does this only occur when using Webp with private files?

I tried adding debugging code to the generation method, and it fires when the image is first uploaded, but not when the image is requested again.

I'm not able to reproduce the problem in 🐛 Webp derivatives not refreshed when files stored in sub-directory Needs review any more. Maybe there was a particular combination of versions and patches I was using that caused it. I've struck through my comment above so it doesn't cause confusion

https://git.drupalcode.org/project/webp/-/merge_requests/39, rebases with 8.x-1.x branch so that the .diff patch can be used with rc2 version module.

Still waiting for steps to reproduce.