Don't offer dithering options when converting to 16 bit, it doesn't
make much sense to dither for anything but 8 bit. Thanks to Elle for
testing that this assumption is indeed true.
Use a properly commented #define instead of just a hardcoded "8 bits"
to make clear that this is a pure GUI "restriction".
Keep only one menu item per precision, and have two separate radio
menu items for gamma/linear. Also add gamma/linear radio buttons to
the precision conversion dialog.
