WPF - Add lock to AbstractRenderHandler Dispose
WPF - Add lock to AbstractRenderHandler Dispose

Was trying to avoid adding a lock, unfortunately it's still possible for the CopyMemory function to be called
after the MemoryMappedFile has been Disposed.

Livit/CefSharp,Livit/CefSharp,Livit/CefSharp,Livit/CefSharp