Keep FBOs around in EffectChain again.

Seemingly creating and deleting them is crazy expensive on NVidia
(~3 ms for a create/delete pair), so 6dea8d2 caused a performance
regression at high frame rates. Now we instead keep one around per
context (they cannot be shared), which brings us basically back
to where we were performance-wise.

Reported by Christophe Thommeret.