New Feature: Infinite scrolling text

[norihiro]

This is an idea to keep scrolling infinite text.

• [ ] Set a limit of lines, returns number of characters processed until reaching the limit.
• [ ] Or, limit/fix text height and scroll until it reaches the bottom.
• [ ] Add an option to lock update callback until rendering is done, and not render text until there is a pending text to scroll-in.
• [ ] Bugfix that a scrolling text suddenly vanishes if a new text is empty.