XCB fullscreen output selection
The XCB window provider should support listing and selecting a custom output for fullscreen mode (like Wayland does).
Implementing the fullscreen output enumeration and selection is a little bit tricky with X11. The !RandR can enumerate outputs (or configure them, but VLC is not interested in that). However the E.W.M.H. still insists on legacy Xinerama to actually specify the intended output when going fullscreen.