org.gnome.Geary.yaml: Workaround for WebKitGTK not printing under Flatpak

Since WebKitGTK does not currently support printing under Flatpak,
enable RW access to XDG downloads folder, so at least people can print
to a PDF, then print that.

See https://github.com/flathub/org.gnome.Geary/issues/42

org.gnome.Geary.json

@@ -27,6 +27,7 @@
         "--metadata=X-DConf=migrate-path=/org/gnome/Geary/",
         "--filesystem=~/.config/geary:ro",
         "--filesystem=~/.var/app/org.gnome.Geary/config/geary:ro",
+        "--filesystem=xdg-download:rw",
         "--filesystem=/tmp"
     ],
     "cleanup": [

org.gnome.Geary.yaml

@@ -61,7 +61,10 @@ finish-args:
   - "--filesystem=~/.config/geary:ro"
   - "--filesystem=~/.var/app/org.gnome.Geary/config/geary:ro"
 
-  # Let view source keep on working as-sis for now. Bug 779311. */
+  # Workaround for printing to PDF until WebKitGTK supports printing
+  - "--filesystem=xdg-download:rw"
+
+  # Let view source keep on working as-sis for now. Bug 779311.
   - "--filesystem=/tmp"
 
 cleanup: