Hack around FBO/VAO sharability issues.

We have a problem when trying to delete an EffectChain or ResourcePool;
we might have created FBOs or VAOs in the wrong context. Work around it
for now (unbreaking Kdenlive) by making VAOs non-persistent again,
and simply never deleting FBOs (leaking them).

A proper solution here will be hard, unfortunately, and will nede some thought.