Add asymmetrical convolutional filter kernel flipping

From the Wikipedia page for [image processing kernels](https://en.wikipedia.org/wiki/Kernel_(image_processing)#Convolution):

`If the kernel is not symmetric, it has to be flipped both around its horizontal and vertical axis before calculating the convolution as above.`[<sup>[1]</sup>](http://www.songho.ca/dsp/convolution/convolution2d_example.html)