static MPV_HANDLE: OnceLock<usize>Expand description
The mpv handle of the backend this process created, for the video surface.
A OnceLock rather than a field reached through PlayerBackend, because the
trait is cross-platform and a raw mpv pointer is not something every backend
should have to pretend to have. Stored as usize because a raw pointer is
neither Send nor Sync; the only consumer is the GTK main thread, which is
also where mpv was created.
Written once at construction and never cleared: the backend outlives the window, so there is no window in which this could dangle while a surface is still using it.
TRACES: UR-080 | DR-231