vf_sub: fix previous commit

The previous fix breaks another obscure case: if the second vf_sub adds
margins, the image is accidentally not extended, which would return in
an assertion failure when returning the bogus image.
demux_mkv: adjust subtitle preroll again

Revert commit 24e52f66; even though the old beheavior doesn't make sense
(as the commit message assured), it turns out that this works better:
typically, it means preroll will start from the previous video key frame
(the video CUE index will contain clusters with video key frames only),
which often coincides with subtitle changes. Thus the old behavior is
actually better.

Change the code that uses CueDuration elements. Instead of merely
checking whether preroll should be done, find the first cluster that
needs to be read to get all subtitle packets. (The intention is to
compensate for the enlarged preroll cluster-range due to reverting
commit 24e52f66.)