[9fans] resize

[erik quanstrom]

i found a little bug today with one-pixel tracking images.  (yahoo.co.jp, for 
instance.)
i don't think this program is in the distribution.

; diffy -c resize.c
/n/dump/2012/1030/sys/src/cmd/resize.c:17,24 - resize.c:17,24
        bpp = src->depth/8;
        bpl = src->width*sizeof(int);
  
-       qp.x = (ssize.x<<12)/dsize.x;
-       qp.y = (ssize.y<<12)/dsize.y;
+       qp.x = (ssize.x<<12)/(dsize.x==0? 1: dsize.x);
+       qp.y = (ssize.y<<12)/(dsize.y==0? 1: dsize.y);
        _sp.y = sr.min.y<<12;
        for(dp.y=0; dp.y<=dsize.y; dp.y++){
                sp.y = _sp.y>>12;