Always store the preview returned by Imagemagick before resizing it

[oparoz]

For performance reasons, a preview provider class can be modified to return an image of a maximum size which makes sense for a preview in a browser. Some people have talked of max 1024x1024 per example.

But in order to really improve performance, especially for the classes which use Imagemagick, it would be worth for the preview class to store that large image (usually PNG) in the cache before resizing it. That way, the next time a thumbnail is generated, GD can quickly resize that max size cached picture.

[PVince81]

At some point I also thought about having gradual size increases. Instead of having images for ANY pixel sizes, we could have cached images for 32x32, 128x128, 256x256, 1024x1024, etc generated on-demand.

[karlitschek]

@PVince81 we already only cache the thumbnails that are requested. Not every possible size.
So it's the decision of the app which size it requests. This makes sense IMHO

[oparoz]

Introducing the maximum size preview. #13674