Add radius and center to GPUImageGaussianBlurFilter

No performance impact on iphone or ipad noticed
Add GPUGaussianBlurPositionFilter and restore the original code for GPUGaussianBlurFilter
Fix compile errors for new blur filter
Added cancelRecording method to movieWriter

Added simple wrapper for AVAssetWriter cancelWriting based on
movieWriter finishWriting in order to cancel a recording and avoid
flushing the file to disk
Merge pull request #700 from jjxtra/master

New GPUImageGaussianBlurPositionFilter
Merge pull request #731 from chrisben/master

Added cancelWriting method to MovieWriter
Fixed issues with uniforms not updating on multiple instances of the same filter, while still maintaining shader program caching.