Fix build with png-1.5.

--- src/ImageData.cpp.orig	Wed Dec 12 22:07:21 2007
+++ src/ImageData.cpp	Sun Oct 23 11:33:36 2011
@@ -371,6 +371,7 @@ bool ImageData::pngLoadPNGLib (const string& filename)
 
   char         sig[8];           /* PNG signature array */
 
+  unsigned int  w, h;
   int           bit_depth;
   int           color_type;
 
@@ -455,8 +456,10 @@ bool ImageData::pngLoadPNGLib (const string& filename)
   // read all the info up to the image data
   png_read_info(png_ptr, info_ptr);
 
-  png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth,
+  png_get_IHDR(png_ptr, info_ptr, &w, &h, &bit_depth,
                &color_type, NULL, NULL, NULL);
+  width = w;
+  height = h;
 
   // Set up some transforms.
   if (color_type & PNG_COLOR_MASK_ALPHA)
