osx: explicitly request depth and stencil buffer bits

This is a blind patch because I don't know enough about the osx backend
and the osx backend probably doesn't even work these days anyway but
since people have filed bugs specifically on OSX that imply they don't
have a depth or stencil buffer this tries to fix that.

Maybe someone will eventually pick up the osx backend again and verify
if this helps.

http://bugzilla.clutter-project.org/show_bug.cgi?id=1394

clutter/osx/clutter-backend-osx.c

@@ -61,6 +61,8 @@ clutter_backend_osx_post_parse (ClutterBackend  *backend,
    */
   NSOpenGLPixelFormatAttribute attrs[] = {
     NSOpenGLPFADoubleBuffer,
+    NSOpenGLPFADepthSize, 24,
+    NSOpenGLPFAStencilSize, 8,
     0
   };
   self->pixel_format = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs];