Fix scene-viewer.py for use under MacOSX with a Mac App.

User should compile scene-viewer-macosx.app and copy it from the Xcode DerivedData
folder into the top-level scene-viewer source directory.  Then install the Blender
add-on and configure:

Encoder path: /path/to/gameplay-encoder
Viewer path: /path/to/scene-viewer-macosx.app  (doesn't matter if there is trailing /)
Workspace: (leave blank)