When the last camera reader disconnects, cycle the output pipeline
(v4l2sink) through READY → PLAYING to reset v4l2loopback's format
state. Without this, switching between camera apps that use different
PipeWire paths (e.g. Chromium → GNOME Snapshot) fails with "no more
input formats" because v4l2loopback retains stale format state from
the previous session.
The reset is guarded by an input_was_active flag so it only triggers
after an actual streaming session, not during PipeWire's brief device
probes on startup.
Tested on Dell XPS 16 (Panther Lake) with PipeWire 1.6.2:
- Snapshot → Zoom → Snapshot: working
- Suspend → resume → Snapshot: working
- Boot → first Snapshot: 30fps
Gaggery Tsai