Fix build with png-1.5.

--- cimg.cc.orig	Wed Dec 14 23:31:38 2011
+++ cimg.cc	Wed Dec 14 23:32:22 2011
@@ -94,11 +94,11 @@ CImg::CImg(const char *filename) {
       ct == PNG_COLOR_TYPE_GRAY_ALPHA)
     png_set_gray_to_rgb(pngp);
 
-  alloc(pngp->width,pngp->height);
+  alloc(png_get_image_width(pngp, infp),png_get_image_height(pngp, infp));
   if (!ok) { fclose(f); return; }
   ok = 0;
 
-  for(i=0;i<pngp->height;i++) {
+  for(i=0;i<png_get_image_height(pngp, infp);i++) {
     png_read_row(pngp, (png_bytep) (&data[i*rowlen]), NULL);
   }
 
