opj_compress: add a -TargetBitDepth switch for TIFF output (#1384)

Sometimes, given the same (16-bit TIF) input, one wants to generate a variety of J2C outputs (say, 16-, 12-, and 10-bit). This patch allows one to downsample input files, and so makes it easier to automate OpenJPEG in mass generation of J2Cs without having to pipe though an image processing program.