VeNum optimizations to libpng to improve PNG decode time

Set correct counter in neon routine for SUB filter type.
Enable Neon optimizations for all filter types and pixel depths.
cherry picked from commit b912f64bc4bb174fc055cda58e303faaa640b8b1

Change-Id: I4ff1c78637db1ab91689ad90da9b5e7da07d2a9e
Update copyright to The Linux Foundation

Change-Id: I537e53b3bb9145aabb60dc482c495118d1f06b03
external: libpng: fix numerous compiler warnings

Clean up the code for unused variables to handle them
a bit more elegantly.

Change-Id: I8b01c27673128719dad74276599d5a286c4dd412