Significant speed-up rate allocation by rate/distoratio ratio

Significant speed-up rate allocation by rate/distoratio ratio

- Avoid doing 128 iterations all the time, and stop when the threshold
  doesn't vary much
- Avoid calling costly opj_t2_encode_packets() repeatdly when bisecting the
  layer ratio if the truncation points haven't changed since the last
  iteration.

When used with the GDAL gdal_translate application to convert a 11977 x
8745 raster with data type UInt16 and 8 channels, the conversion time
to JPEG2000 with 20 quality layers using disto/rate allocation (
-co "IC=C8" -co "JPEG2000_DRIVER=JP2OPENJPEG" -co "PROFILE=NPJE_NUMERICALLY_LOSSLESS"
creation options of the GDAL NITF driver) goes from 5m56 wall clock
(8m20s total, 12 vCPUs) down to 1m16 wall clock (3m45 total).