imagefreeze: Add support for replacing the output buffer

By default imagefreeze will still reject new buffers after the first one and immediately return GST_FLOW_EOS but the new allow-replace property allows to change this.

Whenever updating the buffer we now also keep track of the configured caps of the buffer and from the source pad task negotiate correctly based on the potentially updated caps.

Only the very first time negotiation of a framerate with downstream is performed, afterwards only the caps themselves apart from the framerate are updated.