Add a new action signal to allow applications to capture
the most recently rendered frame directly from the swapchain
back buffer.
Unlike the existing "last-sample" property, which exposes
the raw input sample before any sink-side processing, this
signal captures the final displayed image after any internal
image processing (e.g., UV remap, color balance, overlay) has been
applied.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9423>
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