CMake: Implement YUZU_USE_BUNDLED_FFMPEG

For Linux, instructs CMake to use the FFmpeg submodule in externals.
This is HEAVILY based on our usage of the late Unicorn.  Minimal change
to MSVC as it uses the yuzu-emu/ext-windows-bin. MinGW now targets the
same ext-windows-bin libraries as MSVC for FFmpeg. Adds FFMPEG_LIBRARIES
to WIN32 and simplifies video_core/CMakeLists.txt a bit.