Frame skip every 11 seconds or so

If I run glxgears under gnome-shell then I see a frame skip almost reliably about every 11 seconds. It's easier to see if glxgears is maximized. But the skip doesn't happen at all using mutter instead of gnome-shell.

This is probably related to the "big hammer" garbage collection in GJS, which can be triggered regularly for reasons outlined in !923 (closed). Even if a fix eventually goes in Glib or GJS, it's probably better to track the issue in gnome-shell since that's where a glitch is actually visible.

Tested with the Nvidia driver. Users of Mesa drivers are likely to hit #4830 which will mask this one.