Turn ImFontLocked into a class

As `ImFontLocked` utilizes a reference counter, changed it to a class so that at worst case we still got the destructor to decrease the reference count.

Dalamud/Interface/ManagedFontAtlas/Internals/FontHandle.cs

@@ -182,7 +182,7 @@ internal abstract class FontHandle : IFontHandle
 
             // Transfer the ownership of reference.
             errorMessage = null;
-            return new(fontPtr, substance.DataRoot);
+            return IFontHandle.ImFontLocked.Rent(fontPtr, substance.DataRoot);
         }
     }