[regression] Vsync is lost (constant graphics tearing) after plugins render effects

When I start compiz it's nice and smooth and there is no video tearing at all. However after certain plugins have rendered their effects, compiz suddenly loses all vsync and the display tears constantly. Compiz doesn't seem to recover either, like it's stuck in indirect rendering mode (or something like that).

TESTCASE 1:
1. Start: compiz --replace composite opengl move resize decor water
2. Verify: No graphics tearing by dragging windows around and playing video.
3. Trigger the water plugin using Ctrl+Super.
4. Observed: As soon as the ripples dissipate, the display tears constantly when dragging windows or playing video. And never recovers.
4. Expected: No graphics tearing when dragging windows around and playing video.

TESTCASE 2:
1. Start: compiz --replace composite opengl move resize decor compiztoolbox switcher
2. Verify: No graphics tearing by dragging windows around and playing video.
3. Trigger the switcher using Alt+Tab
4. Observed: As soon as the switcher goes away and returns the desktop, the display tears constantly when dragging windows or playing video. And never recovers.
4. Expected: No graphics tearing when dragging windows around and playing video.

TESTCASE 3:
1. Start: compiz --replace composite opengl move resize decor wobbly
2. Move a window around triggering the wobbly-ness.
3. Observed: The display tears constantly when dragging windows or playing video
3. Expected: No graphics tearing when dragging windows around and playing video.

I noticed this a while ago but only just confirmed it with lp:compiz-core r2984. It will take me a while to bisect/diagnose further...