Make TeXworks work with host TeX binaries

[stloeffler]

According to https://docs.flatpak.org/en/latest/sandbox-permissions.html#filesystem-access, the host's /usr is mounted under /var/run/host/usr. Adding the relevant bin paths under that folder allows TeXworks to access them (see the currently unused binpaths.patch).

However

• It contradicts the idea of sandboxing
• Dynamic linking (to libraries on the host) does not work, resulting in errors such as

  /var/run/host/usr/bin/pdflatex: error while loading shared libraries: libpoppler.so.73: cannot open shared object file: No such file or directory
  /var/run/host/usr/bin/lualatex: error while loading shared libraries: libtexlua52.so.5: cannot open shared object file: No such file or directory
  /var/run/host/usr/bin/xelatex: error while loading shared libraries: libicuuc.so.60: cannot open shared object file: No such file or directory

• Even when the program starts, it might use incompatible versions of libraries (i.e., those found in the sandbox rather than those on the host)

It would be possible to set LD_LIBRARY_PATH, but doing so globally when invoking TeXworks is not a good idea (for the same reason as above - this could potentially run TeXworks with incompatible host libraries instead of the sandbox libraries). One possibility would be to modify the Tw sources to set LD_LIBRARY_PATH only when running executables from the host (i.e. from somewhere under /var/run/host), but determining the proper settings (at runtime) is probably not trivial.

[LyzardKing]

This works for me: Running the flatpak with the permission --talk-name=org.freedesktop.Flatpak flatpak run --talk-name=org.freedesktop.Flatpak org.tug.texworks and adding the external command as flatpak-spawn --host <command> <args>