[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.)

pngerror.c

@@ -423,7 +423,7 @@ png_set_error_fn(png_structp png_ptr, png_voidp error_ptr,
  * pointer before png_write_destroy and png_read_destroy are called.
  */
 png_voidp PNGAPI
-png_get_error_ptr(const_png_structp png_ptr)
+png_get_error_ptr(png_const_structp png_ptr)
 {
    if (png_ptr == NULL)
       return NULL;