Speed up SetContent by checking length of combining characters before reflect.DeepEqual

[kivattt]

Adding these length checks before the reflect.DeepEqual speeds up SetContent when not drawing Unicode combining characters

With xset r rate 300 255 (key repeating 255 times per second), I recorded myself scrolling through 2000 files in my file manager project where I'm using tview with and without (left) this change:

https://github.com/user-attachments/assets/9a4f8f83-7a8d-4b5f-b368-8687c53e1ad3

(6x sped up)

I believe the performance difference is a lot less dramatic for most projects, since in this old version of my file manager the screen was always unnecessarily being cleared with Box.DrawForSubclass() in tview