Only enumerate cached textures that are modified when flushing. (#918)

* Only enumarate cached textures that are modified when flushing, rather than all of them.

* Remove locking.

* Add missing clear.

* Remove texture from modified list when data is disposed.

In case the game does not call either flush method at any point.

* Add ReferenceEqualityComparer from jD for the HashSet

Ryujinx.Common/ReferenceEqualityComparer.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Ryujinx.Common
+{
+    public class ReferenceEqualityComparer<T> : IEqualityComparer<T>
+        where T : class
+    {
+        public bool Equals(T x, T y)
+        {
+            return x == y;
+        }
+
+        public int GetHashCode([DisallowNull] T obj)
+        {
+            return obj.GetHashCode();
+        }
+    }
+}