Support system-wide installed oxipng and ect

[PaintNinja]

Currently Nicephore relies on its own copies of oxipng and ect by checking the references.json and downloading the appropriate executables for the right OS.

This unfortunately leaves Linux users behind as it's common practice to install through a package manager instead of the traditional author-provided downloads method.

To support Linux, it would be nice if we could somehow check if oxipng and/or ect are available system-wide and use them before attempting to download a copy. A simple "probe" might work (run whereis oxipng or which oxipng and check the returned process error code).