Fix buffer age with intermediate framebuffer rendering
With recent change to per-output multi-refresh rate rendering we can use buffer age on individual outputs. But there are artifacts with wobbly windows effects when an output is transformed and we render to the intermediate framebuffer.
For now we just omit buffer age usage when the intermediate framebuffer is used after 4683b71d. But this comes with a performance penalty, so it would be nice to have buffer age working flawlessly in this case too.