Use fmemopen for JPEG 2000
Use fmemopen for JPEG 2000

This avoids using a temporary file, so it is faster.

The code was already available, but not activated and using a wrong
file mode (which could have resulted in a crash).

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

stweil/leptonica-old,stweil/leptonica-old,stweil/leptonica-old