Update to libpng 1.6.19

Removes all Android modifications, including
SK_PNG_INDEX_SUPPORTED (for subset decoding).

BUG:23265085
BUG:25424175
Change-Id: Iaa4f6cc9fc6db8e4b46a83211363adfcbf07b589
diff --git a/TODO b/TODO
index 63613b8..7263377 100644
--- a/TODO
+++ b/TODO
@@ -6,6 +6,7 @@
 Fix problem with C++ and EXTERN "C".
 cHRM transformation.
 Remove setjmp/longjmp usage in favor of returning error codes.
+Palette creation.
 Add "grayscale->palette" transformation and "palette->grayscale" detection.
 Improved dithering.
 Multi-lingual error and warning message support.