The ScreenCast portal defines 3 cursor modes [1]:
* Hidden: no visible cursor (value: 1)
* Embedded: cursor is drawn in the frames (value: 2)
* Metadata: cursor is sent as stream metadata (value: 4)
The values are power-of-two due to be used as flags.
Explicitly listing these values in an enum improves legibility
of the code, so do that instead of hardcoding 1, 2, and 4.
[1] https://github.com/flatpak/xdg-desktop-portal/blob/main/data/org.freedesktop.portal.ScreenCast.xml#L302-L316
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