image-copy: Use "buffer age" of 1 for capture

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.

src/wayland/handlers/image_copy_capture/render.rs

@@ -225,7 +225,7 @@ where
         // TODO re-use offscreen buffer to damage track screencopy to shm
         0
     } else {
-        session_damage_tracking.age_for_buffer(&buffer)
+        1
     };
     let mut fb = offscreen
         .as_mut()