Loop while there are pictures to encode.

- Start by filtering the first picture through each filter chain, and encode
the result.
- Then drain the first (non-null) filter chain and pass the result through
the other filter chain, and encode the result.
- Then drain the second (non-null) filter chain and pass the result through
the other filter chain, and encode the result.
- And so on, until there is no more filtered pictures to encode.