x11-libs/gtk+: bump to 3.22.9 and apply various fixes

x11-libs/gtk+: bump to 3.22.9 and apply various fixes

* Stop inheriting eutils - epatch isn't used with EAPI-6 and einstalldocs
comes from EAPI-6 itself as well
* Remove cairo[svg] comment as it is a required dep since long ago after
the bug was filed upstream - it is needed by file print backend (print to
file), which is not optional
* Adapt to libepoxy changes that made epoxy/glx.h and related source code
optional and put it behind a USE flag, bug #611710
* Remove unnecessary libXrender dependency (though cairo[X] will still
pull it in), bug #611032
* Remove -O3 to -O2 replacement and strip-flags; apparently people have
been doing that with success, so lets give it a chance after issues that
happened with -O3 over 10 years ago
* Install select docs instead of the default ones, which include an
uninteresting README.win32, README.in source file that for some reason
is shipped in tarball, README.commits that is interesting for git usage
instructions only, and a useless ChangeLog that isn't properly generated
anymore. The DOCS array is declared locally in multilib_src_install_all,
because gnome2_src_install calls "default" which calls einstalldocs too,
which would fail if the DOCS is given in global scope and it sees it, as
the docs won't be found in multilib WORKDIR.

Gentoo-bug: 611032
Gentoo-bug: 611710
Package-Manager: Portage-2.3.3, Repoman-2.3.1