cwebp: add PNG metadata extraction

the values of XMP/EXIF/ICC are currently unused.

Change-Id: I39d78b9a179f2d36c9c8ea12776bfdf6d8d18663
diff --git a/examples/cwebp.c b/examples/cwebp.c
index 47a9c3b..4004a9a 100644
--- a/examples/cwebp.c
+++ b/examples/cwebp.c
@@ -327,7 +327,7 @@
     // If no size specified, try to decode it as PNG/JPEG (as appropriate).
     const InputFileFormat format = GetImageType(in_file);
     if (format == PNG_) {
-      ok = ReadPNG(in_file, pic, keep_alpha);
+      ok = ReadPNG(in_file, pic, keep_alpha, metadata);
     } else if (format == JPEG_) {
       ok = ReadJPEG(in_file, pic);
     } else if (format == TIFF_) {