games-fps/freedoom: Add doom engine runtime dependency

games-fps/freedoom: Add doom engine runtime dependency

Freedoom is distributed by upstream with the intention to be run by a
doom engine; this is also the expectation of end users. This patch adds
a doom engine as a runtime dependency for Freedoom.

Three possible doom engines are listed in RDEPEND: games-engines/odamex,
games-fps/doomsday, games-fps/gzdoom. The games-fps/gzdoom package is
set to preferred as it is the recommendation of the Freedoom upstream
team.

Closes: https://bugs.gentoo.org/687672
Package-Manager: Portage-2.3.67, Repoman-2.3.14
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: James Le Cuirot <chewi@gentoo.org>