[[ ThreadedRendering ]] Changed LockImageFrame to return a retained GImage inside MCGImageFrame rather than a pointer to the internal MCGImageFrame - thus making it more multi-thread friendly.
[[ ThreadedRendering ]] Made Release/Retain of MCImageRep thread-safe
[[ ThreadedRendering ]] Tweaked release methods to ensure thread-safety.
Merge branch 'feature-threaded_rendering' into develop