Reload shared renderers when a file changed on disk
When we have to render a new pixmap, compare the file date with the date of the last time the file was cached. if the dates are different (not only older, in order to allow downgrades) then force the svg renderer to be reloaded from the new file, otherwise the renderer with the old file still loaded will save in cache old graphics with the id derived from the date of the new file, causing a wrong cache entry