GUIScript: let the compiler decide whether to inline functions

gcc with optimisations skips them due to too much use, but
the warning is therefore only visible in release builds
MUSImporter::OpenPlaylist: plug unlikely out-of-bounds access

shuts up a gcc warning; release build builds again
remove another inline keyword, add -Wno-error=inline

(gcc's -Winline is silly at times, see gcc bug 32089)
guiopt: fixed bg1 autopause control issue on some installs