Release v0.19.0

Add an abstraction for rational numbers and fix related type conversion inconsistencies

Use extern statics rather than extern functions to access FFmpeg constants

Update changelog

Allow inlining trivial functions

Use GitHub Actions instead of Travis

Rename the GitHub Actions workflow

Add a GitHub Actions build

Update the changelog

Update the .gitignore

Add video filtering support (#88)

Update the changelog

Handle AVStream side data deprecation

Bump the ac-ffmpeg-features version

Define new FFmpeg features