WINDOW: explicitely use libmpv output backend for mpv embedding

src/module/qt_window/mpvwidget.cc

@@ -42,6 +42,7 @@ MpvWidget::MpvWidget(queue *queue, lkt_db *db, module_reg *reg, bool *launched,
     setFocusPolicy(Qt::StrongFocus);
 
     mpv_set_option_string(mpv, "osc", "yes");
+    mpv_set_option_string(mpv, "vo", "libmpv");
     if (mpv_initialize(mpv) < 0)
         throw std::runtime_error("could not initialize mpv context");