Letter-by-letter text chunk effects

[MyreMylar]

At the minute text effects operate at a whole chunk level or a whole text block level. The type writer effect seems to contradict this, but it is actually an easy hack because we can just not render some letters, there is no complicated 'updating of each letter' process.

To make these work we would need to do some system-level hackery rather than just a new text effect using existing tools.

This was wanted originally for a drop-in letter effect in this issue comment:
#24 (comment)