This avoids the following libass warning when using the subtitles
filter: "Neither PlayResX nor PlayResY defined. Assuming 384x288"
Subtitles tests change because the output is ASS and the PlayRes[XY]
ends up in the output.
Text subtitles packets are not 0-terminated (and if they are,
it is handled by the recoding process since 0 is a valid
Unicode code point). The terminating 0 would overwrite the
last payload octet.
OTOH, packets must be 0-padded.
Fix a problem reported in trac ticket #2431.
