crop(1) -c negative coordinates

[istvan bak <bdhpfl@gmail.com>]

The -c flag in crop(1) works incorrectly for images that have a
nonzero upper left corner X coordinate. It crops the left and right
side of the image relative to [0, width[, instead of [r.min.x,
r.max.x[ . So, images with r.min.x < 0 have their left side eaten away
and their right side kept with the unwanted border. One can make such
images with paint(1). Rectangle coords of images can be viewed with
tweak(1). I presume such rectangles are normal and that images don't
usually need to be normalized (please confirm?).

A way to trigger the bug:
Add wide enough pink border (this works as expected):
crop -i -100 -b 255 150 150 image | page
Remove maximum amount of pink border:
crop -i -100 -b 255 150 150 image | crop -c 255 150 150 | page
Lament the fact that pink border remains on at least one side.

Fix below; apologies if gmail hard wraps long lines.

/sys/src/cmd/crop.c:34,40 - crop.c:34,40
  crop(Memimage *m, ulong c)
  {
        Memimage *n;
-       int x, y, bpl, wpl;
+       int i, x, y, bpl, wpl;
        int left, right, top, bottom;
        ulong *buf;

/sys/src/cmd/crop.c:61,68 - crop.c:61,69
                x = unloadmemimage(m, Rect(m->r.min.x, y, m->r.max.x,
y+1), (uchar*)buf, bpl);
                if(x != bpl)
                        sysfatal("unloadmemimage");
-               for(x=0; x<wpl; x++)
-                       if(buf[x] != c){
+               for(i=0; i<wpl; i++)
+                       if(buf[i] != c){
+                               x = m->r.min.x + i;
                                if(x < left)
                                        left = x;
                                if(x > right)