[devel] Corrected const_png_ in png.h to png_const_ to avoid polluting

 the namespace.  Added png_get_current_row_number and
 png_get_current_pass_number for the
 benefit of the user transform callback.
 Added png_process_data_pause and png_process_data_skip for the benefit of
 progressive readers that need to stop data processing or want to optimize
 skipping of unread data (e.g. if the reader marks a chunk to be skipped.)
diff --git a/scripts/symbols.def b/scripts/symbols.def
index 0868d3f..e793017 100644
--- a/scripts/symbols.def
+++ b/scripts/symbols.def
@@ -222,3 +222,7 @@
  png_get_sCAL_fixed @214
  png_set_background_fixed @215
  png_get_io_chunk_type @216
+ png_get_current_row_number @217
+ png_get_current_pass_number @218
+ png_process_data_pause @219
+ png_process_data_skip @220