c/multi: Make sure to retire delivered frames once the session goes invisible or non-active

Launching two instances of hello_xr will cause an assert in the app pacer.

src/xrt/compositor/multi/comp_multi_compositor.c

@@ -942,6 +942,12 @@ multi_compositor_latch_frame_locked(struct multi_compositor *mc, uint64_t when_n
 	u_pa_latched(mc->upa, mc->delivered.frame_id, when_ns, system_frame_id);
 }
 
+void
+multi_compositor_retire_delivered_locked(struct multi_compositor *mc, uint64_t when_ns)
+{
+	slot_clear_locked(mc, &mc->delivered);
+}
+
 xrt_result_t
 multi_compositor_create(struct multi_system_compositor *msc,
                         const struct xrt_session_info *xsi,

src/xrt/compositor/multi/comp_multi_private.h

@@ -228,6 +228,16 @@ multi_compositor_deliver_any_frames(struct multi_compositor *mc, uint64_t displa
 void
 multi_compositor_latch_frame_locked(struct multi_compositor *mc, uint64_t when_ns, int64_t system_frame_id);
 
+/*!
+ * Clears and retires the delivered frame, called by the render thread.
+ * The list_and_timing_lock is held when this function is called.
+ *
+ * @ingroup comp_multi
+ * @private @memberof multi_compositor
+ */
+void
+multi_compositor_retire_delivered_locked(struct multi_compositor *mc, uint64_t when_ns);
+
 
 /*
  *

src/xrt/compositor/multi/comp_multi_system.c

@@ -257,12 +257,17 @@ transfer_layers_locked(struct multi_system_compositor *msc, uint64_t display_tim
 
 		// The client isn't visible, do not submit it's layers.
 		if (!mc->state.visible) {
+			// Need to drop delivered frame as it shouldn't be reused.
+			multi_compositor_retire_delivered_locked(mc, now_ns);
 			continue;
 		}
 
 		// Just in case.
 		if (!mc->state.session_active) {
 			U_LOG_W("Session is visible but not active.");
+
+			// Need to drop delivered frame as it shouldn't be reused.
+			multi_compositor_retire_delivered_locked(mc, now_ns);
 			continue;
 		}