Optimize apply_png_predictor

[mitchelljkotler]

I have a PDF which takes over 29 minutes to load on my computer, spending all of its time in apply_png_predictor. I noticed some easy ways to optimize that function, which resulted in a load time of less than 4 seconds, for a greater than 450x speed up. The PDF is probably unusual, but I think the optimized version should be helpful for any other similar PDFs. I will open a PR to close this issue with my optimized version of the method.