Fixed computation mistake in GPUImageAverageColor
Fixed computation mistake in GPUImageAverageColor

Wrong pixels were addressed.
sansuiso/GPUImage,silvansky/GPUImage,PlanetaToBe/GPUImage,drewwilson/GPUImage,shi-yan/GPUImage,lacyrhoades/GPUImage,eighteight/GPUImage,UndaApp/GPUImage,tastyone/GPUImage,PlanetaToBe/GPUImage,UndaApp/GPUImage,birthmark/GPUImage,Matzo/GPUImage,bgulanowski/GPUImage,bgulanowski/GPUImage,bgulanowski/GPUImage,YuAo/GPUImage,wysaid/GPUImage,sansuiso/GPUImage,drewwilson/GPUImage,wfxiang08/GPUImage,YuAo/GPUImage,tastyone/GPUImage,BradLarson/GPUImage,wysaid/GPUImage,jsharp83/GPUImage,YuAo/GPUImage,eighteight/GPUImage,tastyone/GPUImage,jsharp83/GPUImage,zakinaeem/GPUImage,birthmark/GPUImage,BradLarson/GPUImage,wfxiang08/GPUImage,jakeva/GPUImage,wysaid/GPUImage,BradLarson/GPUImage,drewwilson/GPUImage,silvansky/GPUImage,eighteight/GPUImage,Matzo/GPUImage,Matzo/GPUImage,shi-yan/GPUImage,jsharp83/GPUImage,jakeva/GPUImage,zakinaeem/GPUImage,sansuiso/GPUImage,zakinaeem/GPUImage,silvansky/GPUImage,lacyrhoades/GPUImage