Compile OpenGL filters on MacOSX

Alexandre Janniauxrequested to merge
alexandre-janniaux/vlc:opengl-filter-osx/1 into master

Fix regression with d98f1346 which made the draw filter unavailable on MacOSX, and expose the glfilter mock on MacOSX.

It would be better fixed as a configure.ac fix, but I'm deferring that to other patchsets I'll send later.

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