Option to use existing derivative

[crisc2000]

Future request:
-Give GThumb+ an option to use an existing derivative instead of building a new one. They may be already similar derivatives builded by the Piwigo core or other extensions.

Example:
SmartPocket theme mobile theme is included by default in the Piwigo 2.9.1 and is also activated by default.
SmartPocket will already build similar derivatives (216px height hardcoded in thumbnails.tpl).
There is no point in building another 200px height (this is the default option) derivative for the GThumb+.

I have written more about this here: http://piwigo.org/forum/viewtopic.php?id=28180

[crisc2000]

I made more tests and it's seems actually SmartPocket theme know how to use existing derivatives made by the Piwigo core (not the custom derivatives)
http://piwigo.org/forum/viewtopic.php?pid=168305#p168305