Added support for looping movies and recording from two simultaneous video inputs

`GPUImageLoopingMovie` is added which supports looping movies. Other than that it behaves exactly the same as `GPUImageMovie`.

Also, `GPUImageTwoInputFilter` is fixed to support recording inputs from two video sources simultaneously (e.g. live camera plus a video overlay) by always taking the `frameTime` from the first input and ignoring the `frameTime` of the second. This issue is documented at http://stackoverflow.com/questions/14184731/gpuimagemoviewriter-frame-presentationtime