Merge upstream fixes into solenv/flatpak-manifest.in

* <https://github.com/flathub/org.libreoffice.LibreOffice/commit/
  f5d97291e302d98c217c955c52b255795d4d4cad> "Bump runtime-version to
  org.gnome.Platform 3.26"

* <https://github.com/flathub/org.libreoffice.LibreOffice/commit/
  7d69e505165d4bbf5fa0b34b5fb2cbff9dfaf9bd> "For now, bundle gst-libav to be
  able to play mp4 videos in impress"

Change-Id: I1924d84be7fe577e302b0c1b8c7e2971e8583d73

solenv/flatpak-manifest.in

 {
     "id": "org.libreoffice.LibreOffice",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "3.24",
+    "runtime-version": "3.26",
     "sdk": "org.gnome.Sdk",
     "sdk-extensions": [
         "org.freedesktop.Sdk.Extension.gcc7",
@@ -32,6 +32,19 @@
                 "/usr/lib/sdk/openjdk9/install.sh"
             ]
         },
+        {
+            "name": "gst-libav",
+            "buildsystem": "meson",
+            "builddir": true,
+            "config-opts": ["-Ddisable_gtkdoc=true"],
+            "sources": [
+                {
+                    "type": "archive",
+                    "url": "https://gstreamer.freedesktop.org/src/gst-libav/gst-libav-1.12.4.tar.xz",
+                    "sha256": "2a56aa5d2d8cd912f2bce17f174713d2c417ca298f1f9c28ee66d4aa1e1d9e62"
+                }
+            ]
+        },
         {
             "name": "libreoffice",
             "sources": [