avfilter/vf_libplacebo: flush render cache when input is invisible

This prevents leaking stale metadata from previous frames, for example if
an overlay temporarily obscures this input and then un-obscures it again. It
is worth pointing out that this does change the semantics subtly, because of
the smoothing period on detected HDR metadata, but I argue that the new
behavior is an improvement, as it will avoid leaking past metadata that is
definitely no longer relevant after an image is unobscured.

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