gnome-base/gnome-desktop: bump to 3.26.2

gnome-base/gnome-desktop: bump to 3.26.2

Also force seccomp when available, or configure fails. This is not
patched to be optional on purpose - this kind of security is NOT
optional as far as I'm concerned.

Compared to gnome overlay version, this one:

* adds a big patchset for more sandboxed thumbnailing fixes,
  memory leak fixes and other misc changes that don't break ABI
* patch to hopefully fix thumbnailing on non-seccomp architectures
  (bwrap was still enabled, leading to broken runtime as bwrap
  requires seccomp)
* RDEPENDs on sys-apps/bubblewrap with seccomp as required
* removes USE=dbus dep on glib
* revbumps over overlay revision to ensure upgrades

Closes: https://bugs.gentoo.org/653786
Signed-off-by: Mart Raudsepp <leio@gentoo.org>
Package-Manager: Portage-2.3.52, Repoman-2.3.11