lavf/concat: document the use of the files durations.

Address trac ticket #2319.
snowenc: add forgotten emms_c()

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

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Move ff_cropTbl and ff_zigzag_direct from dsputil to mathtables.

These are widely used throughout libavcodec, nothing dsputil-specific.

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