Optimize Sub and Paeth filtering

[fintelia]

This PR changes the filter_internal method to make it more conducive to auto-vectorization. Using the benchmark infrastructure from #362, it increases overall encoding performance by ~30%:

before...

Directory                                     Ratio             Encode
---------                                    -------     --------------------
Total                                        28.472%      40 mps   0.14 GiB/s

after...

Directory                                     Ratio             Encode
---------                                    -------     --------------------
Total                                        28.472%      60 mps   0.20 GiB/s

Also fixes a bug I noticed while making this change: StreamWriter's prev_buf would contain filtered image data rather than unfiltered pixels (and adds a test to confirm).

[HeroicKatora]

Looks good to me in terms of implementation, just could use some more comments to capture some information on how it was derived.

[fintelia]

@HeroicKatora I added an explanation of how the alternative Paeth filter implementation was derived.