Fixed the color attachment comparison bug

Fixes the bug of using the wrong conversion table while displaying the video from the camera (GPUImageVideoCamera) or playing back a video using the GPUImageMovie class.

In shorts the comparison between two CFStringRef variables was the cause of the problem.

Instead of using CFStringCompare, there was a == comparison which didn't return the correct result.

What is troubling me the most though is that the exact same bug exists in Apple's sample code of '[Real-time Video Processing Using AVPlayerItemVideoOutput](https://developer.apple.com/library/ios/samplecode/AVBasicVideoOutput/Listings/AVBasicVideoOutput_APLEAGLView_m.html#//apple_ref/doc/uid/DTS40013109-AVBasicVideoOutput_APLEAGLView_m-DontLinkElementID_6)' as well, so be advised :)

The bug caused the red color to be displayed as orange (due to the fact that it was using the 709 conversion table instead of the 601), as seen from the screenshot below:

![comparison](https://f.cloud.github.com/assets/49122/2430943/bc634c16-ad0e-11e3-86de-7fe2ef0eefbb.jpg)