libx264encoder: Ensure stream size is always a multiple of 2
- libx264encoder: Ensure stream size is always a multiple of 2
libx264 apparently rejects any stream that is not a multiple of 2. So ensure we adjust the stream size to that. This also inserts a pad filter to ensure we pad with black rather than garbage.
- encoder: Make it possble to override the filter graph used for software encode
We may want to adjust the filter graph in an encoder to insert additional filters. Rather than overriding the full method for creating the filter graph, which would lead to a lot of duplication, this simply exposes the string that is passed to FFmpeg for parsing.