Hiding Error/Placeholder Tiles

[InsertUser]

When editing the map it if often useful to zoom in past the maximum resolution available for some aerial imagery to either get more precise mouse placement or avoid snapping to undesired geometry. Some imagery providers seems to simply return an error message when a zoom level doesn't exist, but other return "error" tiles that overlay useful but blurry zoom levels with useless but sharply focused placeholder images.

As the resolution of the available imagery varies from place to place it is often not possible to simply set an appropriate "max zoom" level for that imagery provider.

It would be useful to have a setting to define tiles that should not be displayed if they are downloaded from the server and for which a lower zoom level should be substituted.

I have checked both Mapbox and Mapquest Open Aerial "placeholder tiles" in my local cache and all the error tiles from these providers consistently hash to the same values so it may be relatively easy to filter these out at some stage of the process.

[Don-vip]

Similar to #5819 and #10454

[wiktorn]

Fixed in 8344/josm and [o31124]. See #10454