screencast: support for capturing areas

[warningnonpotablewater]

Currently the screencast portal only supports capturing monitors and windows, but it can also be useful to support capturing arbitrary areas of the screen(s). Such a feature could be implemented using mutter's RecordArea screencast function for xdg-desktop-portal-gtk and using slurp for xdg-desktop-portal-wlr.

[matthiasclasen]

I'd be curious what the use case is, but if you want to provide a patch that adds and handles a suitable source type for that, why not ?

[SeaDve]

I think the use case is similar to screenshot, sometimes you don't want to record the entire window, just a section of a window, without having to crop it using video editors.

Possibly recording a video meeting, without the ui elements, only the actual meeting.

[swick]

The application receiving the video stream can crop it at will. I think you'll have to explain why cropping the stream on the compositor instead is advantageous.

[warningnonpotablewater]

Consider the following case: the client wants to record an area than spans several monitors (such as, for example, the whole stage consisting of every monitor connected). In order to do that, the client needs to be aware of the logical placement of the monitors on the stage. While it's possible to do it manually, it's extremely painful and unreliable since a simple rearrangement of the monitors will break the whole thing.

[vchernin]

See also emersion/xdg-desktop-portal-wlr#154