Fix issue #1451 "GPUImageMovie is leaking with initWithPlayerItem ?"

Patch for issue https://github.com/BradLarson/GPUImage/issues/1451

Comments: 
+ (CADisplayLink *)displayLinkWithTarget:(id)target selector:(SEL)sel
so the dealloc of GPUImageMovie method is never called.

https://developer.apple.com/library/ios/documentation/QuartzCore/Reference/CADisplayLink_ClassRef/Reference/Reference.html#//apple_ref/occ/clm/CADisplayLink/displayLinkWithTarget:selector:

In order to release the target it's enough to call [displayLink invalidate] in -(void)endProcessing method.