sixel: erase: fix clearing of cell->attrs.clean

When erasing a sixel, the cells underneath it must be marked as
'dirty', in order to be re-rendered.

This was not being done correctly; the for loop loops *from* the start
col, meaning the *end* col is *not* sixel->pos.col, as that's
the *number* of columns, not the *end* column.

sixel.c

@@ -180,7 +180,7 @@ sixel_erase(struct terminal *term, struct sixel *sixel)
 
         row->dirty = true;
 
-        for (int c = sixel->pos.col; c < min(sixel->cols, term->cols); c++)
+        for (int c = sixel->pos.col; c < min(sixel->pos.col + sixel->cols, term->cols); c++)
             row->cells[c].attrs.clean = 0;
     }