games-fps/prboom-plus: Bump to version 2.5.1.5.4540.1

games-fps/prboom-plus: Bump to version 2.5.1.5.4540.1

The non-free USE flag is removed because non-free assets are no longer
provided in the source tarball. The png USE flag is removed since
sdl2-image provides that functionality now. The sdl-image and sdl-mixer
USE flags are renamed to sdl2-image and sdl2-mixer respectively to match
the use of the matching SDL2 libraries now.

Closes: https://bugs.gentoo.org/703610
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Ulrich Müller <ulm@gentoo.org>