The p_previous_pic keeps the picture that was returned by Filter(), which is
the basis for the extra pictures created during the drain, by incrementing by
one "framerate tick" each output. We need to keep a reference as the output
picture might be released by the caller, before the drain is called.

We keep the original date of the incoming picture so we can compare to the
"framerate" ticks to know if we need extra pictures or not.

When expanding the frame rate, we have the following output:
- pic0 -> original date
       -> original date + tick 1
- pic1 -> original date + tick 2
       -> original date + tick 3
- pic2 -> original date + tick 4
       -> original date + tick 5

The date of the incoming picture is always included in the last output
frame date + duration.

The code assumes that enough drain calls will be done until the next filter
call. (A flush in between is good as well)

We no longer return pictures chained using vlc_picture_chain_AppendChain().