Revert "More changes in FBO handling"

This reverts commit 56e5c91617c77fb2882adcbe71e018ff63a600ce.
Revert "Switched FBOs to be stored per RenderTarget instead of only one on the GraphicsDevice (improves performance on Linux nVidia drivers)"

This reverts commit 5162d6b4c0f9ce148bc11cc6336ebfe31f0c98c0.