system: Provide libpng decoding benchmark

[joebonrichie]

There are two options that i know of:
https://raw.githubusercontent.com/glennrp/libpng/libpng16/contrib/examples/pngtopng.c
gcc -Ofast pngtopng.c -o pngtopng -lpng

and
https://raw.githubusercontent.com/joebonrichie/benchmarks/master/pngbench.c
gcc -Ofast pngbench.c -o pngbench -lpng

The user will need the libpng development package installed (e.g. libpng-devel) to link to a benchmark executable if applicable.

Typically testing on a large complex image such as: https://upload.wikimedia.org/wikipedia/commons/6/6a/PNG_Test.png, will produce the biggest differences.

At a system level, libpng decode performance will typically be improved from the following

• swapping out the canonical zlib library for a faster variant e.g. zlib-ng
• Building with --enable-hardware-optimizations=yes
• x86_64: building with x86_64-v2 or higher (note: this will wipe out any gains from --enable-hardware-optimizations=yes)
• Possibly building with PGO (openmadriva does it but I've been unable to replicate a speedup)