[WIP] Implement native litr-muxers module

[IanDBird]

Work In Progress: This is not yet ready to be merged but is to provide visibility of the progress of a NativeMediaTarget

This PR introduces the following:

• A separate litr-muxers module
• The build scripts and process required to build ffmpeg's libavformat with a number of common muxers (including parsers and bitstream filters)
• Code that wraps libavformat's muxers and exposes them via an interface similar to Android's MediaMuxer.
• A NativeMediaMuxerMediaTarget that allows LiTr to use these new classes to mux output samples into the target file.

Status

While most of the module skeleton is complete, along with build scripts/introductions, the main area remaining in the JNI interface. We need to flesh out MediaMuxer.cpp more along with dealing with the marshalling of required data via NativeMediaMuxer.cpp

[IanDBird]

All associated changes have been split off into separate PRs and merged individually!