The logic `age_for_buffer` used seems to be a misinterpretation of the
protocol.
The wording is a little unclear, but it seems tracking buffer age is the
responsibility of the client, and the client is required to accumulate
damage and pass it in `damage_buffer`.
Our clients initially weren't doing that correctly. I updated
xdg-desktop-portal-cosmic to use `damage_buffer` after testing on
wlroots, and cosmic-workspaces was recently updated as well.
e1342fb2e3