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/tests/pngvalid-gamma-threshold b/tests/pngvalid-gamma-threshold
index 2cfca91..3fe4edf 100755
--- a/tests/pngvalid-gamma-threshold
+++ b/tests/pngvalid-gamma-threshold
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngvalid --gamma-threshold
+exec ./pngvalid --strict --gamma-threshold