fpng plugin for vapoursynth

[mikewando]

I've been dumping PNGs a lot lately and was frustrated with how slow imwri was to do it. So I poked around and managed to get fpng working as an alternative. So far seems to work well and much faster in my testing.

https://github.com/Mikewando/vsfpng

Also includes a python script in examples/dump_frames.py which could be interesting for folks who have the same usecase.