Fix Zoomify to display retina tiles

[crubier]

This is a small PR that changes a few lines in zoomify source:

• allows to use retina tiles in Zoomify
• fixes HiDPI / retina / pixelRatio zoom levels for tiled image sources (Zoomify, etc.) #7795
• adds an option in the Zoomify example to display retina tiles

[crubier]

Before:

After:

[crubier]

This is a small PR, how long can we expect before review, merge and release ? Thank you :-) 🙏

[crubier]

@tschaub or @ahocevar , let me know if there is anything I can do to help you expedite this :-)

[tschaub]

@crubier - minor point, but it would be nice to squash all those commits into fewer that represent sensible chunks of work. It looks like this work could be a single commit.

As a general practice, if you git pull --rebase openlayers master you can have your work rebased on the latest from the hosted master branch without introducing merge commits. There are some good docs online about squashing commits if you haven't done that before.

[crubier]

@tschaub Ok thank you, done

[crubier]

@tschaub up :-)

[crubier]

👍

[jameshoward]

Just in case anyone runs into the same issue as me, the only reason the example works when setting tileSize to 256 / pixelRatio is because the example image server effectively ignores the tile group directory. For example, http://vips.vtech.fr/cgi-bin/iipsrv.fcgi?zoomify=/mnt/MD1/AD00/plan_CHU-4HD-01/FOND.TIF/TileGroup1/2-0-0.jpg and http://vips.vtech.fr/cgi-bin/iipsrv.fcgi?zoomify=/mnt/MD1/AD00/plan_CHU-4HD-01/FOND.TIF/TileGroup100/2-0-0.jpg return the same image.

If you try to use the same method using an image from another Zoomify source (e.g. the converter), you will likely get 404 errors when trying to load tiles because the tile grouping calculations are different between the generated image and the ZoomifySource.

[crubier]

@jameshoward did you find a solution to this? I was not aware of this problem, only tried on the demo (The server of the demo image is down now actually...)

[ahocevar]

The remaining issue was fixed with #10562.

[jameshoward]

@crubier No, I didn't find a solution other than to tweak the generator that was creating my tiles to output them at 256x256. The solution is #10562 looks great though!