GL: gstglcontext_cocoa.h missing from 1.12

[Gottfried Haider]

Hello,

I saw that commit 2a440f60 [1] did away with the public
gstglcontext_cocoa.h header in gst-plugins-bad.

In prior versions of GStreamer I was using this header to enable GL
context sharing on macOS - see the __APPLE__ define in my code for
what exactly [2]. Do you have a suggestion for how I would
forward-port this to 1.12?

Thanks in advance,
Gottfried

[1] https://github.com/GStreamer/gst-plugins-bad/commit/2a440f60e400137ae2b5b267b6cc75239eda545e#diff-6f6590adafc17b596261de5d82d5df3f
[2] https://github.com/gohai/processing-glvideo/blob/master/src/native/impl.c