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.Graphics.Gpu/Engine/MethodCopyTexture.cs

@@ -94,7 +94,7 @@ namespace Ryujinx.Graphics.Gpu.Engine
                 srcTexture.HostTexture.CopyTo(dstTexture.HostTexture, srcRegion, dstRegion, linearFilter);
             }
 
-            dstTexture.Modified = true;
+            dstTexture.SignalModified();
         }
     }
 }