FBOs never released

While looking into QTBUG-65080 I came across another strange thing: FBOs are never actually released, it seems. Any QRenderTarget in a QRenderTargetSelector will lead to creating a new FBO that is then alive for ever.

The expectation is (IMHO) that destroying a QRenderTarget leads to destroying the FBO (or at least making it free for reuse) internally.

If the issue in QTBUG-65080 gets fixed, the situation will become more acute since one may then need QRenderTargets (and thus FBOs) just to be able to do a BlitFramebuffer. Now, while one may attempt to work this around by reusing QRenderTargets, but that can get problematic atm due to QTBUG-64757 since one cannot change the texture in a QRenderTargetOutput once set...