Doc: update video processing tutorial code for OpenCV v2.4.9 and v3a

The code from the [OpenCV iOS video processing tutorial](http://docs.opencv.org/trunk/doc/tutorials/ios/video_processing/video_processing.html) is outdated and crashes at runtime. 

See this:
http://answers.opencv.org/question/40249/colorcpp3380-error-215-scn-3-scn-4-depth-cv_8u-in/

This pull request addresses this issue.