- API wise, opj_codec_set_threads() can be used on the encoding side
- opj_compress has a -threads switch similar to opj_uncompress

Update the bench_dwt utility to have a -decode/-encode switch

Measured performance gains for DWT encoder on a
Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz (4 cores, hyper threaded)

Encoding time:
$ ./bin/bench_dwt -encode -num_threads 1
time for dwt_encode: total = 8.348 s, wallclock = 8.352 s

$ ./bin/bench_dwt -encode -num_threads 2
time for dwt_encode: total = 9.776 s, wallclock = 4.904 s

$ ./bin/bench_dwt -encode -num_threads 4
time for dwt_encode: total = 13.188 s, wallclock = 3.310 s

$ ./bin/bench_dwt -encode -num_threads 8
time for dwt_encode: total = 30.024 s, wallclock = 4.064 s

Scaling is probably limited by memory access patterns causing
memory access to be the bottleneck.
The slightly worse results with threads==8 than with thread==4
is due to hyperthreading being not appropriate here.

…functional change)

…ith original image

…void potential division by zero

…ecoding side

…id messing up with stepsize (no functional change)

…tandard

The previous constant opj_c13318 was mysteriously equal to 2/K , and in
the DWT, we had to divide K and opj_c13318 by 2... The issue was that the
band->stepsize computation in tcd.c didn't take into account the log2gain of
the band.

The effect of this change is expected to be mostly equivalent to the previous
situation, except some difference in rounding. But it leads to a dramatic
reduction of the mean square error and peak error in the irreversible encoding
of issue141.tif !

…res in test suite

"bench_dwt -I" time goes from 2.8s to 2.2s

Nothing in code analysis nor test suite shows that this margin is
needed.
It dates back to commit dbeebe7
where vector 9x7 decoding was introduced.

"bench_dwt -I" time goes from 2.2s to 1.5s

…the horizontal pass

…est in vertical pass

…the vertical pass

`bench_dwt -encode` times goes from 7.9s to 1.7s

`bench_dwt -I -encode` times goes from 8.6s to 2.1s

…ata organization

~ 9% speed improvement seen on 10980x10980 uint16 image, T36JTT_20160914T074612_B02.tif
opj_compress time from 17.2s to 15.8s