FFmpeg plugin: Fix null pointer reference
Fix potential uninitialized variable

if (data->output->flags & AVFMT_RAWPICTURE)

If this was true, the 'ret' variable would be used without
initialization.
CoreAudio: Set default audio settings on creation

device_id would end up being null, which would mess with the functions.
CoreAudio: If null device id, just set to default
Initial implementation of a v4l2 capture plugin