Add gamescopegrab PipeWire frontend

[flibitijibibo]

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Feature Request

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The PipeWire backend currently has two methods to get a stream node: You can request it via an XDG Portal (portalgrab) or ask KWin directly for the screencast node (kwingrab). gamescope is different in that it just spits out a raw node and doesn't implement any portal functionality; presumably Steam Remote Play is just grabbing this node and running with it:

https://github.com/ValveSoftware/gamescope/blob/master/src/pipewire.cpp

It's unlikely that gamescope will add support for portals since it's not a "real" desktop environment, so this is likely the best way to stream embedded sessions without using kmsgrab.

Related gamescope ticket: ValveSoftware/gamescope#2126