Hint user to enable WebP if webp imagery is trying to display

[aharvey]

WebP support for imagery was added in #14361 however if you add a WebP layer it will silently fail, even then with display errors on it's still unclear what the exact issue is and solution.

I suggest when JOSM receives a WebP layer as indicated by the content-type header, to display a message about how to enable WebP (or better yet enable it by default 😜)

[anonymous]

In fact I see no reason not to enable it by default, and it causes problems for inexperienced users, since they install the plugin and "it doesn't work".

For the "webp" plugin they installed it and that's all, right now they need a manual with the multiple steps about how to enable it.

[stoecker]

In its current form the imageio plugin is in no way ready to be included into core. That's probably also the reason why Taylor didn't do it.

[gaben]

There are also ​problems with the WebP implementation. The old plugin does not produce as many artefacts.