snap: Remove superfluous files from package

Remove all files from snap that are already included in the base snap
or in any connected content snaps.

The main advantage is the massively reduced snap size:
From ~70 MB down to ~6 MB.

pkg/snap/snap/snapcraft.yaml

@@ -26,7 +26,7 @@ apps:
     extensions: [gnome-3-28]
     plugs: [opengl, unity7, home, removable-media, gsettings, network]
     environment:
-      __EGL_VENDOR_LIBRARY_DIRS: $SNAP/usr/share/glvnd/egl_vendor.d
+      __EGL_VENDOR_LIBRARY_DIRS: $SNAP/gnome-platform/usr/share/glvnd/egl_vendor.d:$SNAP/usr/share/glvnd/egl_vendor.d
   cli:
     command: usr/bin/solvespace-cli
     plugs: [home, removable-media, network]
@@ -71,6 +71,14 @@ parts:
       - libglibmm-2.4-1v5
       - libpangomm-1.4-1v5
       - libsigc++-2.0-0v5
-      - libglew2.0
-      - libegl-mesa0
-      - libdrm2
+  cleanup:
+    after: [solvespace]
+    plugin: nil
+    build-snaps: [core18, gnome-3-28-1804]
+    override-prime: |
+      # Remove all files from snap that are already included in the base snap or in
+      # any connected content snaps
+      set -eux
+      for snap in "core18" "gnome-3-28-1804"; do  # List all content-snaps and base snaps you're using here
+        cd "/snap/$snap/current" && find . -type f,l -exec rm -f "$SNAPCRAFT_PRIME/{}" \;
+      done