invalidate arrange when document is changed.
invalidate arrange when document is changed.

This fixes not rendering with single line of text.
AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,danwalmsley/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,danwalmsley/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit,AvaloniaUI/AvaloniaEdit