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