In file-psd, add support for exporting high bit-depth images.  This
is currently limited to 16-bit images, since 32-bit images seem to
have a different structure (our loading code can successfully load
32-bit images exported by the plug-in, but not actual 32-bit PSD
files saved in Photoshop.)  Higher bit-depth images are saved as
16-bit for now.

Note also that when saving a linear image with a built-in linear
profile the result is wrong (the image is exported with a linear-
TRC profile, but the data is perceptual), but this is a general
problem we have to fix, not restricted to the PSD plug-in.