roll libpng to 1.6.37

Adds NEON accelerated palette expansion.

Change-Id: I8a1378c58175c2343541a69b2bd7a1f476931a99
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208580
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>