glInvalidateFramebuffer problem

My engine is cross platform, including directx 11 support, and there there's no concept of a frame buffer, you just attach render targets to 0,1,2.. slots. I find this approach much more natural. So I've built the render target management around that concept. Also I have a lot of post process effects, for which most of the time I need different kind of render targets attached. Creating an FBO for each combination would result in a lot of FBO'S created. Also it would make the render targets have to be strictly attached to those FBO'S. It would apply some restrictions about what render targets I can use and what not. But with my approach, I allocate render targets on demand when I need them, I keep them in a pool of render targets, once an effect is finished, then I mark render target as available for reuse, so it can be used for another post process effect. This way there's no restriction on what render targets can be used for a post process effect, I just reuse the first one available that matches desired resolution and format.