don't check ADLER32 checksums on libpng load

can be rather chatty, and even cause loops on some libpng versions

see https://github.com/libvips/libvips/discussions/2930

libvips/foreign/vipspng.c

@@ -301,6 +301,13 @@ read_new( VipsSource *source, VipsImage *out,
 		PNG_SKIP_sRGB_CHECK_PROFILE, PNG_OPTION_ON );
 #endif /*PNG_SKIP_sRGB_CHECK_PROFILE*/
 
+	/* Don't verify ADLER32 checksums (this can produce a lot of
+	 * warnings).
+	 */
+#ifdef PNG_IGNORE_ADLER32
+	png_set_option( read->pPng, PNG_IGNORE_ADLER32, PNG_OPTION_ON );
+#endif /*PNG_IGNORE_ADLER32*/
+
 	/* Disable CRC checking in fuzzing mode. Most fuzzed images will have
 	 * bad CRCs so this check would break fuzzing.
 	 */