Fix png decoding on x86.

Fix png decoding on x86.

Line sizes are only 8-byte aligned, so use unaliged loads
for add_bytes_l2 pointers.
Increasing the alignment requirement to 16 seemed a bit extreme
(png may be used for rather small sizes).
Also fix a mov that had its arguments swapped, leading
add_bytes_l2 being applied on up to 8 bytes too few.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>