Re-enable Skia in Flatpak builds

...as requested at
<https://github.com/flathub/org.libreoffice.LibreOffice/issues/213> "Can Skia be
reenabled in the Flatpak build?" and implemented on Flathub for now with
<372d036e91>
"Re-enable Skia".  (This partially reverts
9d88f11de5 "Flatpak: Upgrade to 21.08 runtime,
disable Skia".)

Change-Id: I50039d431fd3e6ba4c0d859d71e42e744433806b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148054
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

distro-configs/LibreOfficeFlatpak.conf

@@ -1,5 +1,4 @@
 --disable-odk
---disable-skia
 --enable-release-build
 --with-ant-home=/run/build/libreoffice/ant
 --with-extra-buildid=Flatpak

solenv/flatpak-manifest.in

@@ -606,6 +606,13 @@
                     "dest": "external/tarballs",
                     "dest-filename": "@LIBATOMIC_OPS_TARBALL@"
                 },
+                {
+                    "url": "https://dev-www.libreoffice.org/src/@SKIA_TARBALL@",
+                    "sha256": "@SKIA_SHA256SUM@",
+                    "type": "file",
+                    "dest": "external/tarballs",
+                    "dest-filename": "@SKIA_TARBALL@"
+                },
                 {
                     "url": "https://dev-www.libreoffice.org/src/@DRAGONBOX_TARBALL@",
                     "sha256": "@DRAGONBOX_SHA256SUM@",