allow huge png metadata chunks

By default, libpng refuses to open files with a metadata chunk larger than
8mb. Disable this sanity check.

See: lovell/sharp#1664

libvips/foreign/vipspng.c

@@ -71,6 +71,8 @@
  * 	- support png8 palette write with palette, colours, Q, dither
  * 25/8/18
  * 	- support xmp read/write
+ * 20/4/19
+ * 	- allow huge metadata
  */
 
 /*
@@ -260,8 +262,13 @@ read_new_filename( VipsImage *out, const char *name, gboolean fail )
 
 	/* Read enough of the file that png_get_interlace_type() will start
 	 * working.
+	 *
+	 * By default, libpng refuses to open files with a metadata chunk 
+	 * larger than 8mb. png_set_chunk_malloc_max() disables this sanity
+	 * check.
 	 */
 	png_init_io( read->pPng, read->fp );
+	png_set_chunk_malloc_max( read->pPng, 0 );
 	png_read_info( read->pPng, read->pInfo );
 
 	return( read );