Main thread blocked after resizing window

If a client window is resized while a renderthread animation is running, the GUI thread will get blocked for long periods. The time seems to increase fairly linearly with the number of resize events.

To reproduce: run the example qtdeclarative/quick/shadereffects in Weston. Resize the window a bit, and then try to interact with the sliders. Note that the animations are smooth, but the sliders move sluggishly