libobs: Add audio control functions

This adds a new library of audio control functions mainly for the use in
GUIS. For now it includes an implementation of a software fader that can
be attached to sources in order to easily control the volume.
The fader can translate between fader-position, volume in dB and
multiplier with a configurable mapping function.
Currently only a cubic mapping (mul = fader_pos ^ 3) is included, but
different mappings can easily be added.

Due to libobs saving/restoring the source volume from the multiplier,
the volume levels for existing source will stay the same, and live
changing of the mapping will work without changing the source volume.

libobs/CMakeLists.txt

@@ -190,6 +190,7 @@ set(libobs_util_HEADERS
 
 set(libobs_libobs_SOURCES
 	${libobs_PLATFORM_SOURCES}
+	obs-audio-controls.c
 	obs-avc.c
 	obs-encoder.c
 	obs-service.c
@@ -204,6 +205,7 @@ set(libobs_libobs_SOURCES
 	obs-scene.c
 	obs-video.c)
 set(libobs_libobs_HEADERS
+	obs-audio-controls.h
 	obs-defs.h
 	obs-avc.h
 	obs-encoder.h