Generate the FBO anew every render.

Generate the FBO anew every render.

This fixes an issue where reusing the same EffectChain from two different GL
contexts would cause errors even if they are shared, because FBOs cannot be
shared between contexts (in the ARB extension, anyway).

There might be some negative performance implications to this, but I was unable
to measure any on Intel/Mesa. If this should prove to be a problem in the future,
we could maybe keep one around per context, or have the caller invalidate the
FBO for us.