Steps to reproduce: 1. Enable WebRender. 2. Go to https://immersion.media.mit.edu/demo and wait for it to load and stop animating. 3. Click one of the circles in the graph. Expected results: The animation should be smooth. Actual results: The animation is not smooth. Paint flashing makes it look like we have a large number of blob images. It would be nice to find out why we have so many and what's inside them; if it's mostly transparent pixels, maybe we can do better.