Release GIL for filters

Created by: ArtanisCV

It seems that currently Filter Graph does not release GIL when pushing / pulling filtered frames.

Is it possible to release GIL? The API for Filter Graph is quite complicated, and I am not sure where I can manually add the "with nogil" statement. Maybe here https://github.com/mikeboers/PyAV/blob/develop/av/filter/context.pyx#L107 ?