ui/vnc: optimize setting in vnc_dpy_update()

static void vnc_dpy_update(DisplayChangeListener *dcl,

                           int x, int y, int w, int h)

{

    int i;

    VncDisplay *vd = container_of(dcl, VncDisplay, dcl);

    struct VncSurface *s = &vd->guest;

    int width = surface_width(vd->ds);

    int height = surface_height(vd->ds);

    h += y;

    /* round x down to ensure the loop only spans one 16-pixel block per,

       iteration.  otherwise, if (x % 16) != 0, the last iteration may span

       two 16-pixel blocks but we only mark the first as dirty

    */

    /* this is needed this to ensure we updated all affected

     * blocks if x % VNC_DIRTY_PIXELS_PER_BIT != 0 */

    w += (x % VNC_DIRTY_PIXELS_PER_BIT);

    x -= (x % VNC_DIRTY_PIXELS_PER_BIT);

    x = MIN(x, width);

    y = MIN(y, height);

    w = MIN(x + w, width) - x;

    h = MIN(h, height);

    h = MIN(y + h, height);

    for (; y < h; y++) {

        for (i = 0; i < w; i += VNC_DIRTY_PIXELS_PER_BIT) {

            set_bit((x + i) / VNC_DIRTY_PIXELS_PER_BIT, s->dirty[y]);

        }

        bitmap_set(s->dirty[y], x / VNC_DIRTY_PIXELS_PER_BIT,

                   DIV_ROUND_UP(w, VNC_DIRTY_PIXELS_PER_BIT));

    }

}