The renderer now accesses the zDirection member directly if it exists.
This also has the advantage of not advertising any object methods that
hav no use for most tile sources.
The zDirection addition to the Zoomify tile source only takes effect through
the renderer, so these two renderer cases are added to test the behavior.
zoomify-no-zdirection expects the default behavior.
zoomify-zdirection expects the tiles for the next better resolution.
Via zDirection, the renderer's selection of a tile source's resolution can be
controlled if the view's resolution does not match any tile source resolution.
Currently the zDirection option is only available for Zoomify. The comment
https://github.com/openlayers/openlayers/issues/9343#issuecomment-474341033
describes why caution is advised when using zDirection for other sources.
ol/source/Tile~TileSource#getZDirection has been added to avoid having the
getZDirection method solely in the Zoomify source - just to document the
possibility.
Lutz Helm