[libpng15] Fixed 'unused' warnings and mergeed with libpng16 pngvalid.

Various warnings revealed by configurations added to the tested
configurations in libpng 1.6 need corresponding fixes in libpng 1.5.
This takes the libpng 1.6 pngvalid.c and adds it back to libpng 1.5,
however one write-only test is currently disabled because it does not
work; possibly because of failures to reset or initialize the deflate
stream correctly in write-only cases.

pngwutil.c

@@ -315,6 +315,7 @@ png_zlib_release(png_structp png_ptr)
 
       if (ret != Z_OK)
       {
+#ifdef PNG_WARNINGS_SUPPORTED
          png_const_charp err;
          PNG_WARNING_PARAMETERS(p)
 
@@ -349,6 +350,7 @@ png_zlib_release(png_structp png_ptr)
 
          png_formatted_warning(png_ptr, p,
             "zlib failed to reset compressor: @1(@2): @3");
+#endif
       }
    }