x11-misc/xscreensaver: make deps on gdk-pixbuf, png and gtk optional

* add previously dropped USE="gdk-pixbuf png gtk". It was verified that
  package builds fine and usable without those (individually or all
  together) and none of the dependencies are getting enabled
  automagically. All possible functionality loss is documented via
  greadme. Note that RESTRICTED_USE for gtk were not transfered from the
  old versions because it was verified that package builds and runs fine
  without them.
* document with greadme issue with USE=-jpeg reported by ./configure
* add a couple more perl dependencies found in scripts
* move media-gfx/fbida to optfeatures and elaborate on alternatives and
  other dependencies:
  - it's only one screensaver (vidwhacker) that depends on
    media-gfx/fbida and now it orderly fails to start rather than
    unexpectedly segfault as it was in #790599 times
  - as an alternative to media-gfx/fbida it is possible to use
    imagemagick's convert, which is much more common
  - besides media-gfx/fbida it requires media-libs/netpbm (which is not
    a very common package either) and a tool from libjpeg.
* minor cleanup

Closes: https://bugs.gentoo.org/961204
Signed-off-by: Alexander Golubev <fatzer2@gmail.com>
Part-of: https://github.com/gentoo/gentoo/pull/43429
Closes: https://github.com/gentoo/gentoo/pull/43429
Signed-off-by: Florian Schmaus <flow@gentoo.org>

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