ok i've raised this directly upstream with the mesa-dev team on freedesktop.org and also done a little investigation, and found that the processes are hanging waiting in a poll in libxcb. the mesa-dev team asked everyone who is affected *AND NOT* affected to report which version of DRI is being used in their xorg.conf file (2 or 3) and also which version of mesa is installed.
if that information could be sent here to 848895 then i can refer the mesa-dev team to this bugreport. i'm using DRI2 and mesa 13.0.6 btw if someone familiar with the debian bugtracker system could also add the magic "control affects openscad" to this bugreport that would be great. l.