With the updated PNG library (1.6.8), we're getting stderr warnings about
non-conformance issues with images in the build.  These warnings include:

  * iCCP: known incorrect sRGB profile
  * iCCP: profile 'ICC Profile': 1000000h: invalid rendering intent
  * iCCP: cHRM chunk does not match sRGB

These were normalized by running an ImageMagick 'convert -strip' pass
over each PNG image.  'compare -metric AE' reports 0 delta for all
before/after pairs.