All code interacting with Objective-C objects should now use Automated
Reference Counting rather than manual memory management or Garbage
Collection. Because ARC prohibits C-structs from containing
references to Objective-C objects, all such fields are now typed
'gpointer'. Setting and gettings Objective-C fields on such a
struct now uses explicit __bridge_* calls to tell ARC about
object lifetimes.
https://bugzilla.gnome.org/show_bug.cgi?id=777847
1. Set the sync point after the (possible) upload has occured
2. Wait in the correct GL context (the draw context)
Note: We don't add the GL sync meta to the input buffer as it's not
writable and a copy would be expensive.
Similar to the change with the same name for glimagesink
