Add support for retina aerial imagery

[zdila]

TMS URL parameters {@2x} and {r} are a great addition to support retina background imagery for rendered maps (maps with labels, icons and other symbology). But iD should also support retina for pure aerial (non-hybrid) maps. These maps don't need any extra parameter in URL and to achieve sharp rendering it is sufficient to take 4 adjanced tiles from zoom+1 and place them to the same space as would be for the original tile.

Leaflet supports this by setting zoomOffset to 1 and tileSize to 128. Locus maps on Android also supports this feature.

If you have retina/HDPI display then you can compare the result visually:

• On https://www.openstreetmap.org/edit?editor=id#map=15/48.7313/21.2544 select Ortofotomozaika SR as the background
• Compare it with https://www.freemap.sk/?map=15/48.731300/21.254400&layers=Z which uses the same background

There would still need to be a flag for every background to specift if it is suitable for this (ie. non-hybrid aerial imagery). Unfortunatley I found nothing like this in the documentation of the project defining background maps for iD and I think it should be added there.