Snapshot thumbnails have a very bad quality in 6.4.6

Since our upgrade to JIRA v6.4.6 we noticed that the thumbnail quality is very bad :

I noticed you changed the thumbnail algorithm (similar issue raised in JRA-43200), but the new algorithm made the thumbnail un-usable (cf attached pictures : it seems that the thumbnail is just picking 1 pixel over 4 pixels... so on this example the result is horrible).

Please at least provide us a workaround, or a switch to fall back to the previous quality.