lavfi/scale: allocate interlaced scalers only if needed.

Fix "Value 0.000000 for parameter 'srch' out of range"
error message when source or destination height is 1.

Note: since the av_opt_set_int() calls are not checked for
failure and the interlaced scalers are not actually used,
this error has no consequence apart from a frightening message
in the log.
lut3d: Fix reading 3dl files with leading comments.

Fixes ticket #2787.
lavf/concat: Never fail for sample aspect ratio 0:1.

Fixes ticket #2456.

Reviewed-by: Nicolas George
Fix pix_fmt detection in the native jpeg2000 decoder.

Based on b7a928b by Michael Bradshaw.
Fixes ticket #2683.

Reviewed-by: Nicolas Bertrand