dither: fix integer multiplication overflow that caused crashes.

Fixes: #38 (CVE-2018-20546)
Fixes: #39 (CVE-2018-20547)

caca/dither.c

@@ -6,8 +6,8 @@
  *  This library is free software. It comes without any warranty, to
  *  the extent permitted by applicable law. You can redistribute it
  *  and/or modify it under the terms of the Do What the Fuck You Want
- *  to Public License, Version 2, as published by Sam Hocevar. See
- *  http://www.wtfpl.net/ for more details.
+ *  to Public License, Version 2, as published by the WTFPL Task Force.
+ *  See http://www.wtfpl.net/ for more details.
  */
 
 /*
@@ -116,7 +116,7 @@ enum color_mode
 struct caca_dither
 {
     int bpp, has_palette, has_alpha;
-    int w, h, pitch;
+    size_t w, h, pitch;
     int rmask, gmask, bmask, amask;
     int rright, gright, bright, aright;
     int rleft, gleft, bleft, aleft;