return the encoded block size rather than the raw YUV size

Because all pages except last are -1

A completely translucent teletext subtitle is sent to clear the old
subtitle, unfortunately my zvbi patch (f520484d) messed that use case
up, this one fixes it.

Thanks to Andre De Deudwaerder for finding the problem and tracking it down.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Fixes #5178

Fixes #2973

refs #9350

Make sure the buffer was allocated with enough channels.
This should not happen anymore since transcode now restarts
filter chain on format changes.
Reported-by: William Blough <blough@shell.cs.fsu.edu>

fruity.avi

too fast :/

Close #9223