Add preliminary output/encoder interface

- First, I redid the output interface for libobs.  I feel like it's
  going in a pretty good direction in terms of design.

  Right now, the design is so that outputs and encoders are separate.
  One or more outputs can connect to a specific encoder to receive its
  data, or the output can connect directly to raw data from libobs
  output itself, if the output doesn't want to use a designated encoder.
  Data is received via callbacks set when you connect to the encoder or
  raw output.  Multiple outputs can receive the data from a single
  encoder context if need be (such as for streaming to multiple channels
  at once, and/or recording with the same data).

  When an encoder is first connected to, it will connect to raw output,
  and start encoding.  Additional connections will receive that same
  data being encoded as well after that.  When the last encoder has
  disconnected, it will stop encoding.  If for some reason the encoder
  needs to stop, it will use the callback with NULL to signal that
  encoding has stopped.  Some of these things may be subject to change
  in the future, though it feels pretty good with this design so far.
  Will have to see how well it works out in practice versus theory.

- Second, Started adding preliminary RTMP/x264 output plugin code.

  To speed things up, I might just make a direct raw->FFmpeg output to
  create a quick output plugin that we can start using for testing all
  the subsystems.
Updated comments for outputs/encoders

Changed the comments to properly reflect the new callbacks, as I had
forgotten to update the comments for them both.

Also, changed "setbitrate" and "request_keyframe" return values to be
boolean.
Use macros to improve safety loading callbacks

Just a minor fix mostly because I noticed that I kept accidentally
forgetting to add checks to the code properly.  This is one of those
cases where macros come in useful, as macros can automate the process
and help prevent these mistakes from happening by accident.
Add preliminary ffmpeg plugin (still testing)

- Added some code for FFmpeg output that I'm still playing around with.
  Right now I'm just trying to get it to output to file and try to
  understand the FFmpeg/libav APIs.  Hopefully in the future this plugin
  can be used for any sort of output to FFmpeg.

- Fixed a cast warning in audio-io.c with size_t -> uint32_t

- Renamed the 'video_info' and 'audio_info' structures to
  'video_conver_info' and 'audio_convert_info' to better represent their
  actual purpose, and to avoid confusion with 'audio_output_info' and
  'video_output_info' structures.

- Removed a few macros from obs-def.h that were at one point going to be
  used but no longer going to be used (at least for now)