Add AVFoundation video writer

[tom-seddon]

The macOS version uses ffmpeg for video writing, if it's available at compile time. It should use the system AVFoundation library instead.

--Tom

[tom-seddon]

See also #190

[tom-seddon]

And when the dylib bundler pulls all the dependencies in, the app can end up a bit large.

 36K	./b2 Debug.app/Contents/_CodeSignature
102M	./b2 Debug.app/Contents/libs
5.4M	./b2 Debug.app/Contents/MacOS
3.5M	./b2 Debug.app/Contents/Resources
111M	./b2 Debug.app/Contents
111M	./b2 Debug.app

 36K	./b2.app/Contents/_CodeSignature
118M	./b2.app/Contents/libs
4.5M	./b2.app/Contents/MacOS
3.5M	./b2.app/Contents/Resources
126M	./b2.app/Contents
126M	./b2.app```