gdImageRectangle: support any relative points

Contrary to gdImageFilledRectangle(), gdImageRectangle() has the
documented limitation that the given points have to be the upper
left and the lower right corner, respectively.  However, libgd
already caters to upper right / lower left pairs, and not
catering to the other two combinations seems to be an oversight.

Fixes #177.

src/gd.c

@@ -2262,12 +2262,13 @@ BGD_DECLARE(void) gdImageRectangle (gdImagePtr im, int x1, int y1, int x2, int y
 	}
 
 	if (y2 < y1) {
-		int t;
-		t = y1;
+		int t = y1;
 		y1 = y2;
 		y2 = t;
+	}
 
-		t = x1;
+	if (x2 < x1) {
+		int t = x1;
 		x1 = x2;
 		x2 = t;
 	}