Add a "screenshot-taken" signal from the screenshot service's internal C
implementation, and use that to trigger the camera flash visual effect
and the click sound, allowing them to run in parallel with the PNG
compression instead of waiting until the file is complete to start.
This significantly improves perceived latency on high res setups such as
4K, 5K, or dual 4K screens.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/512
Co-authored-by: Brion Vibber <bvibber@wikimedia.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1658>
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