Update libpng to 1.2.38 to patch a minor security issue.
diff --git a/TODO b/TODO
index a5f6395..face765 100644
--- a/TODO
+++ b/TODO
@@ -22,3 +22,4 @@
 Use greater precision when changing to linear gamma for compositing against
   background and doing rgb-to-gray transformation.
 Investigate pre-incremented loop counters and other loop constructions.
+Add interpolated method of handling interlacing.