community/snapshot: Default to 720p instead 1080p

Unfortunately Snapshot does not yet allow selecting the resolution or setting a default, choosing 1080p (1920x1080) by default. This works well on most modern laptops but is too much for many slower devices, in particular most pmOS phones. Reduce the default to 720p, which works out significantly better in many cases.

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Related:

• community/[gst-plugins-good|gtk4.0]: Backport r... (!79110 - merged)
• https://gitlab.gnome.org/GNOME/snapshot/-/merge_requests/228