Optimize the PNG files.

For documentation purposes, the following sequence of commands was used:

find . -iname "*.png" -print0 | xargs -0 optipng -zc1-9 -zm1-9 -zs0-3 -f0-5
find . -iname "*.png" -print0 | xargs -0 advpng -z3
find . -iname "*.png" -print0 | xargs -0 advpng -z4 -i50
find . -iname "*.png" -print0 | xargs -0 advdef -z3 -i255
find . -iname "*.png" -print0 | xargs -0 advdef -z4 -i50

Note that advpng may not always compress a PNG file (there are some
unsupported formats), but even then, advdef always recompresses them.

With current versions of adv{png,def}, the level 3 is 7zip's implementation
of the DEFLATE algorithm, while level 4 is Google's zopfli implementation.