multimedia/qt6-multimedia: respect upstream defaults

Expose auto-detected dependencies as options to make builds deterministic.
Disabling all audio backends is forbidden to avoid crashes as upstream
doesn't test a configuration that cannot happen on Android, macOS, Windows,
QNX or desktop Linux (except Gentoo).

PR: 268876
Approved by: portmgr blanket, maintainer timeout (partially: no patch but no help)