(struct fringe_bitmap): Change member bits from char to

(struct fringe_bitmap): Change member bits from char to
short to facilitate 16 bits wide bitmaps.  Modify all standard
bitmaps accordingly.
(BYTES_PER_BITMAP_ROW, STANDARD_BITMAP_HEIGHT): New macros.
(FRBITS): Use STANDARD_BITMAP_HEIGHT instead of just sizeof.
(draw_fringe_bitmap): Ditto.
(init_fringe_bitmap) [MAC_OS]: Don't bitswap.
(init_fringe_bitmap) [HAVE_X_WINDOWS]: Enhance bitswapping to
handle up to 16 bits wide bitmaps.
(Fdefine_fringe_bitmap): Doc fix.  Handle wider bitmaps.