Tested on Debian Jessie 64bit:
1) Install Downfall and The Cat Lady via Steam
2) Open the the data-dir of both install-folders, eg.
in your favorite filebrowser
3) rename the libs32 and libs64-directories of Downfall to eg. libs32.old and libs64.old
4) copy over the complete libs32 and libs64-folders from The Cat Lady to Downfall
5) if you want to double scale the screen-size, open the file /your/path/steamapps/common/Downfall/Downfall in a text-editor and append "--gfxfilter hq2x" on the execution line, like:
... "$SCRIPTPATH/data/ags64" "$@" "$SCRIPTPATH/data/" --gfxfilter hq2x
There are 2 lines like that, depending if you run the game in 32 or 64 bits.
Source: Steam Forum