struct x_output): Add left_fringe_width, right_fringe_width,

struct x_output): Add left_fringe_width, right_fringe_width,
and fringe_cols fields.
(FRAME_FRINGE_BITMAP_WIDTH, FRAME_FRINGE_BITMAP_HEIGHT): Remove macros.
(FRAME_X_FRINGE_COLS): Use fringe_cols field.
(FRAME_X_FRINGE_WIDTH): Use fringes_extra field.
(FRAME_X_LEFT_FRINGE_WIDTH): Use left_fringe_width field.
(FRAME_X_RIGHT_FRINGE_WIDTH): Use right_fringe_width field.
(x_compute_fringe_widths): Add prototype.