vout_osx: fix performance issue if machine includes more than one GPU (close #7993)

modules/video_output/macosx.m

@@ -487,6 +487,7 @@ static void OpenglSwap (vlc_gl_t *gl)
         NSOpenGLPFAAlphaSize, 8,
         NSOpenGLPFADepthSize, 24,
         NSOpenGLPFAWindow,
+        NSOpenGLPFAAllowOfflineRenderers,
         0
     };