Allow /bin install directory to be overriden. Debian for example uses this to install FlightGear to /usr/games/ instead of /usr/bin/ https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
Various stgmerge enhancements to make it functional: - Now runs against an entire tile directory, reading each stg file - optionally optimizes the mesh (untested) - outputs to a second directory - control over the size of merged meshes
