Provide ability to choose zoom level of tiles

[wiktorn]

Till now zoom level of imagery was set by JOSM.

User should be able to set whether he wants to work with:

1. high-quality small tiles
2. with low-quality big tiles

First mode is useful when working with aerial imagery to provide highest quality of the image, second is useful, when working on high-dpi screens (retina-like) and imageries where some text information is provided

[wiktorn]

In 8751/josm:

Introduce zoom offseting. See: #11856

• MemoryTileCache size based on ZOOM_OFFSET
• getBestZoom called on every paint, as cost of callign getBestZoom is neglible and clarifies the code
• use ZOOM_OFFSET to choose higher or lower zoom levels
• provide settings panel for ZOOM_OFFSET control

[aceman]

Also imagery at different zooms may be shot at different times. User may want to trace the newer images so will need to "pin" that zoom level of the imagery, but still zoom in in JOSM to place nodes precisely.

[wiktorn]

Replying to aceman:

Also imagery at different zooms may be shot at different times. User may want to trace the newer images so will need to "pin" that zoom level of the imagery, but still zoom in in JOSM to place nodes precisely.

This can be easily achieved using min/max zoom in imagery definition. min/max zoom are available for WMS too now.

[Don-vip]

In 15918/josm:

fix #18720 - see #11856 - Automatically enable "Tile zoom offset" in HIDPI mode