The cached texture was treated as user_data passed to GstGLBaseMemory
and freed with a GDestroyNotify function. However, this data must
be treated specially: it must be destroyed in the GL thread.
https://bugzilla.gnome.org/show_bug.cgi?id=778434
This commit introduces IOSGLMemory which is a GLMemory that falls back to
GstAppleCoreVideoMemory for CPU access. This is a temporary solution until
IOSurface gets exposed as a public framework on iOS and so we can use
IOSurfaceMemory on both MacOS and iOS.
https://bugzilla.gnome.org/show_bug.cgi?id=769210
