Fix video reset and apply new video settings

This allows the changing of bideo settings without having to completely
reset all graphics data.  Will recreate internal output/conversion
buffers and such and reset the main preview.
Implement a few more audio options/functions

Implement a few audio options in to the user interface as well as a few
inline audio functions in audio-io.h.

Make it so ffmpeg plugin automatically converts to the desired format.

Use regular interleaved float internally for audio instead of planar
float.
Separate source activation for main/aux views

Split off activate to activate and show callbacks, and split off
deactivate to deactivate and hide callbacks.  Sources didn't previously
have a means to know whether it was actually being displayed in the main
view or just happened to be visible somewhere.  Now, for things like
transition sources, they have a means of knowing when they have actually
been "activated" so they can initiate their sequence.

A source is now only considered "active" when it's being displayed by
the main view.  When a source is shown in the main view, the activate
callback/signal is triggered.  When it's no longer being displayed by
the main view, deactivate callback/signal is triggered.

When a source is just generally visible to see by any view, the show
callback/signal is triggered.  If it's no longer visible by any views,
then the hide callback/signal is triggered.

Presentation volume will now only be active when a source is active in
the main view rather than also in auxilary views.

Also fix a potential bug where parents wouldn't properly increment or
decrement all the activation references of a child source when a child
was added or removed.
Require restart for audio changes (for now)

Resetting audio while libobs is active is a real pain.  I think I'm just
going to do audio resetting later, or maybe just require restart
regardless just because having to shut down audio streams/lines while
there's sources currently active requires recreating all the audio
lines for each audio source.  Very painful.

Video fortunately is no big deal, so at least there's that.
Rename scene signals

the signals for scenes could have potentially conflicted with default
source signals.  "remove" should be used for source removal, for
example.  Changed the scene signals to "item-add" and "item-remove" for
its items.
Add more checks for NULL pointers
Make ffmpeg test output sync A/V properly

FFmpeg test output wasn't make any attempt to sync data before.  Should
be much more accurate now.

Also, added a restart message to audio settings if base audio settings
are changed.
Fix stereo output bug with ffmpeg test output
Merge pull request #3 from jp9000/master

Merge branch 'master' of https://github.com/jp9000/obs-studio