FIX: use full image for lightbox zoom when thumbnail aspect ratio differs

[small-lovely-cat]

For these steps:

Using a grid to cover two posts, and manually set the width of these two photos, like:

[grid]
![IMG_2938|345x265](upload://bM6E67BaB4LjgBYEZfZ74Uc0jXd.jpeg)
![IMG_2939|345x265](upload://4qZSYjyvDA7ldGQqHjLwU6sAoYr.jpeg)
[/grid]

(The original size of each image is 690*265)

The root cause of this is that the ratio of the thumbnail differs greatly from that of the full-sized image, and lightbox failed to handle that, causing a flicker.

Bug can be seen from clicking the two images below:

A PR has been raised here:

https://github.com/discourse/discourse/pull/37284

[awesomerobot (Kris)]

Thanks for the contribution! the PR has been merged and we’ll see how it goes.