Reinstate proper FFmpeg license for all files.
avcodec/pngdec: Fix padded alloc code with threads

Fixes Ticket2903
Fixes out of array write
no releases should be affected by this

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: fix last_row_size type

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