[libpng16] Work around for duplicate row start calls; added warning messages.

This turns on PNG_FLAG_DETECT_UNINITIALIZED to detect app code that
fails to call one of the 'start' routines (not enabled in libpng-1.5
because it is technically an API change, since it did normally work
before.)  It also makes duplicate calls to png_read_start_row (an
internal function called at the start of the image read) benign, as
they were before changes to use png_inflate_claim. Somehow webkit is
causing this to happen; this is probably a mis-feature in the zlib
changes so this commit is only a work-round.

pngrutil.c

@@ -4105,6 +4105,17 @@ png_read_start_row(png_structrp png_ptr)
 
    png_debug(1, "in png_read_start_row");
 
+   /* Because init_read_transformations, below, modifies values in png_struct
+    * it will not always work correctly if called twice.  This error detects
+    * that condition but just warns, because it does tend to work most of the
+    * time.
+    */
+   if (png_ptr->flags & PNG_FLAG_ROW_INIT)
+   {
+      png_warning(png_ptr, "unexpected duplicate call to png_read_start_row");
+      png_ptr->zowner = 0; /* release previous claim */
+   }
+
 #ifdef PNG_READ_TRANSFORMS_SUPPORTED
    png_init_read_transformations(png_ptr);
 #endif