tiling: Don't completely empty buffer in animation

Previously, this would lead to flickering if the decoder was too slow.
Now, the last frame will be shown until a new one is available.

Closes #382

build-aux/flatpak/org.gnome.Loupe.Devel.json

@@ -159,7 +159,7 @@
                 {
                     "type": "git",
                     "url": "https://gitlab.gnome.org/sophie-h/glycin.git",
-                    "commit": "4b0759839322fad9277c7f823438abe4937e4027"
+                    "commit": "9701eb7ac36cd85ce75241bf50dfe325b50f54c9"
                 }
             ]
         },