The source package that contains glamor_render.c is actually "xorg-server" and not simply "xorg", sorry (I'm a newbie).
Also, to be more specific about the things that trigger the crash, one of them is the progress box that pops up when the "Send" button is pushed in icedove. It makes no difference whether message being sent is a forward or something written from scratch. As stated above, another thing that triggers the bug is firefox-esr's download dialog. A reliable way to reproduce the crash is to run firefox-esr and click where it says "download patch" here: https://patchwork.freedesktop.org/patch/135036/ At any rate, that patch definitely fixes all the issues. I got the xorg-server source package and added the four lines into glamor_render.c as indicated in the patch. Then I ran dpkg-buildpackage, which to my surprise generated several (20!) deb packages. I only installed the two that I already had in my system: xserver-common and xserver-xorg-core, and all problems went away after a reboot. If it's not a "stretch", please try to include Olivier's patch before stretch goes stable. Cheers.
