FindPNG.cmake: find library files with specific versions first.

Find library files with specific version numbers in the name (e.g. libpng16.a or libpng17.so) first, and only look for the more generic name (e.g. libpng.so) afterwards.

Use case: vcpkg (https://github.com/Microsoft/vcpkg) builds a file called 'libpng16.a' on Linux (similar on respective platforms). On many distributions, incl. Ubuntu, the library provided by system package managers is called 'libpng.so'. This leads to a situation where, when using vcpkg, it finds the vcpkg-installed headers but the system provided library. Changing the search order fixes this.