Cross compiling via CMake & multichannel detection

[himwho]

This rather large PR combines a few things:

• cross compiling via Cmake with proper ffmpeg dep handling tested on macos and windows (linux tbd)
• syntax fixes for windows
• multichannel audio detection on input files
• better module linking for ffmpeg
• more readme for compiling

I can imagine this would bring up some questions with compiling via non-cmake and happy to adjust as needed

[ffAudio]

Wow, thank you so much for all the work. Unfortunately there seems something gone amiss with merging and rebasing.
Could you please try to rebase your work on top of master so we don't get all those merge commits?

And another thing, a week ago I started refactoring the module layout in this branch:
https://github.com/ffAudio/foleys_video_engine/tree/file-layout

The reasoning was also adding cmake, building on the CI and having the examples built, so we have some kind of testing.

I also added some code snippets that hopefully lead to a version that can use the OS media frameworks like AVFoundation and alike.

Would it be possible to work on that one instead? I cannot promise though, when I can merge the file-format branch into master. I would like to have it compile first, but maybe that is somehting that's getting easier with your work?

Thanks again and keep up the good work!

[himwho]

I will try to break it up into little 2 or 3 PRs instead

As for the cmake stuff, it works on macos, windows (tested) but to do so it has to pull pre-built bins for ffmpeg from 2 different sources; however its pretty streamlined in the cmakelist so ill make that the last PR in case you decide you want to do it differently

[himwho]

@ffAudio ok i split off the multichannel and syntax stuff to this PR: #20

i will evaluate the new cmake structure you have and see if my contribution is still warranted or otherwise switch to what you merged in!

thanks!