doc/filters: remove another instance of '×'

Fix trac ticket #2247.
lavfi/select: avoid double -> int lossy conversion in debug log
lavfi/select: correctly set prev_t and prev_pts

In particular, fix trac ticket #2248.
matroskadec: support seek to non keyframes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf: add seek2any to allow forcing seeking to non keyframes via AVOptions

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>