synfigstudio: boneyard

Main reasons for boneyard:

* As this formula now stands, it fails to build (with a mysterious
  error) against FFmpeg 3.0 (#49178);

* This is a GUI-only tool (look at its awkward test), so it doesn't
  quite belong to core to begin with; and due to the build issue above,
  work is needed if it is to be revived in homebrew/gui;

* There's not enough interest, seeing that we are two releases or half a
  year behind, and no user even proposed a version bump;

* Upstream's preferred method of installation is the app bundle (which
  seems to be a much more complete solution, given the app bundle's
  size), distributed in binary disk images.

More discussions in Homebrew/homebrew#49178.

Closes Homebrew/homebrew#49378.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>