Use VST 2.x plugins as audio filters in OBS.

• http://teragonaudio.com/article/How-to-make-your-own-VST-host.html
• http://www.reaper.fm/sdk/vst/vst_ext.php
• https://forum.juce.com/t/mac-64-bit/6295/5
• https://gist.github.com/t-mat/206e3e7dfc3f89421bc1
• https://github.com/audacity/audacity/blob/17afc51644b2b327e173a23d6066dde598838c03/src/effects/VST/aeffectx.h

In general VST 2.4 is platform independent. There are only three platform dependent opcodes :
effEditOpen
audioMasterGetDirectory
audioMasterOpenFileSelector

Here are the required API changes for 64 bit Mac OS X:

effEditOpen:
the [ptr] argument is a WindowRef on 32 bit Mac.
On 64 bit this is a NSView pointer. The plug-in needs to add its own NSView as subview of it.

audioMasterGetDirectory:
the [return value] is a FSSpec on 32 bit Mac.
On 64 bit this is a char pointer pointing to an UTF-8 encoded string.

audioMasterOpenFileSelector:
the VstFileSelect struct uses FSSpec's on 32 bit Mac.
On 64 bit Mac these are char pointers pointing to UTF-8 encoded strings.