using unsigned for height and width cause a lot of confusion. I grab

X's header and see that XImage use int for its width and height so
I change RImage to follow that.

wrlib/wraster.h

@@ -190,7 +190,7 @@ enum RImageFormat {
  */
 typedef struct RImage {
     unsigned char *data;	       /* image data RGBA or RGB */
-    unsigned width, height;	       /* size of the image */
+    int width, height;	       /* size of the image */
     enum RImageFormat format;
     RColor background;		       /* background color */
 } RImage;