Point to correct ffmpeg location by default

[merlijn-sebrechts]

As mentioned on the forum, OpenToonz can't find ffmpeg in the snap. The code expects

• either a config option pointing to the directory containing ffmpeg
• or ffmpeg being in the current working directory. (The comment in the source incorrectly assumes this is the root directory of OpenToonz; that is only the case on Windows, not on Linux).

So the only good option in order to have ffmpeg work by default in this snap is to configure the path of ffmpeg in the preferences.

This PR does that in two ways:

• It adds a default preferences.ini which will be used to populate user preferences when the application first starts up. This, however, will not fix the issue for existing installations of this Snap.
• It adds the ensure-defaults.sh scripts to insert/modify the ffmpeg path in existing user preferences. This, however, will not fix the issue for new installations of this Snap.

Both of these combined fix the issue for both new and existing installations of this snap.

To test this, install the snap, run it, close the "opentoonz startup" dialog and choose the menu item Render > Fast Render to MP4. You should see a box "the command cannot be executed because the scene is empty" instead of "ffmpeg not found".

[merlijn-sebrechts]

Note: I added a final commit removing the ffmpeg layout since it's apparently not used by OpenToonz.

[ghost]

Awesome stuff, I'll rebuild the snap. We can do the same for opentoonz-morevna as well I guess?