Mixdown functionality.
* Mixdown possible via libsndfile and ffmpeg!
* Fixed some ffmpeg deprecation warnings
* Mixdown UI only shows working Container, Codec and Format combinations!
* Minor bugs and warnings fixed
Implemented basic audio animation.
* AnimatableProperty: Propper cache writing and spline interpolation for reading (the solution for stair steps in audio animation)
* Animatable properties so far are: volume, pitch, panning
* Users note: Changing the pitch of a sound results in wrong seeking, due to the resulting playback length difference.
* Users note: Panning only works for mono sources, values are in the range [-2..2], this basically controls the angle of the sound, 0 is front, -1 left, 1 right and 2 and -2 are back. Typical stereo panning only supports [-1..1].
* Disabled animation of audio related ffmpeg output parameters.
* Scene Audio Panel: 3D Listener settings also for Renderer, new Volume property (animatable!), Update/Bake buttons for animation problems, moved sampling rate and channel count here
ui/ --> startup/bl_ui
op/ --> startup/bl_operators
scripts/startup/ is now the only auto-loading script dir which gives some speedup for blender loading too.
~/.blender/2.56/scripts/startup works for auto-loading scripts too.
