applemedia/gl: Update code to use ARC

applemedia/gl: Update code to use ARC

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