Allow to zoom more in the panorama view

The current maximum zoom level is often not enough to clearly see some distant features of a picture.

Commit 7f1e17c9 implements a first solution to increase zoom: upscale the original image, and generate an additional set of tiles from this upscaled version. However, it really doesn't scale: tile generation has to manipulate really large images (more than 4 GB RAM for large panorama), and it produces a hell of a lot of tiles. And this is for just one additional level of zoom.

We should instead find a solution where the browser itself upscales the tiles when we need more zoom than the base level.