Expose high performance session state

Author: peppy

osu.Desktop/Performance/HighPerformanceSessionManager.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Runtime;
+using System.Threading;
 using osu.Framework.Allocation;
 using osu.Framework.Logging;
 using osu.Game.Performance;
@@ -11,6 +12,8 @@ namespace osu.Desktop.Performance
 {
     public class HighPerformanceSessionManager : IHighPerformanceSessionManager
     {
+        public bool IsSessionActive => activeSessions > 0;
+
         private int activeSessions;
 
         private GCLatencyMode originalGCMode;

osu.Game/Performance/IHighPerformanceSessionManager.cs

@@ -14,6 +14,11 @@ namespace osu.Game.Performance
     /// </summary>
     public interface IHighPerformanceSessionManager
     {
+        /// <summary>
+        /// Whether a high performance session is currently active.
+        /// </summary>
+        bool IsSessionActive { get; }
+
         /// <summary>
         /// Start a new high performance session.
         /// </summary>