API: deprecate 'bpp' member in favor of 'prec'

API: deprecate 'bpp' member in favor of 'prec'

in opj_image_comp and opj_image_comptparm structures.

bpp was redundant with prec, and almost never set by the library, except
by opj_image_create(). This change should hopefully not impact existing,
working, users of the API, which should already have used prec to get
things working.

Fixes #1379