Support for frozen apps about imageio

[paulmueller]

This one can be closed now (#270).

[paulmueller]

I propose the following workflow:

1. Implement an entry point named e.g. imageio_download_binary imageio_download_bin for downloading the required binary.
2. Add the option --package-dir to the entry point which will download the binary to the plugins recources directory of the installed package. This directory can be determined with pkg_resources.recource_filename (which also works with eggs), e.g. pkg_resources.resource_filename("imageio.plugins", imageio.plugins.ffmpeg.FNAME_PER_PLATFORM[imageio.plugins.ffmpeg.get_platform()]).
3. Give the binary file in the plugins directory highest priority when calling the get_exe method.
4. For Pyinstaller, only a simple hooks script is required:

   # hook-imageio.py
   from PyInstaller.utils.hooks import collect_data_files
   datas = collect_data_files('imageio')

   (needs pull request for PyInstaller)

This workflow as these advantages:

1. It does not break the current behaviour
2. Not many changes to the sources of imageio are required
3. It allows to download binaries with a command line script
4. On CIs such as appveyor or travisCI, it allows to easily build PyInstaller distributables with only the required binaries

[almarklein]

Sounds like a plan. How do we deal with the fact that there are multiple binaries? Would you do e.g.
imageio_download_binary ffmpeg ?

[paulmueller]

Yes, that would be the usage of the entry point script. Did I oversee anything? I could start with a PR.

[almarklein]

As far as I can see, this should work. Looking forward to a PR!

[paulmueller]

I have a question regarding the _set_platform_resources method in setup.py (used in build_with_fi). What is the use-case of this method? Also, there are two lines dealing with ffmpeg commented out in _set_platform_resources?

[almarklein]

The purpose was to make it possible to build dists with the libraries packaged in. It's not used anymore though (at least not by me). The FFMPEG lines were added at some point, but then commented because it would make the dist package very large.