Redo the text buffer implementation.

[NQNStudios]

The code storing the text for the text buffer is pretty scary to me. Each line is stored as a fixed-width C-string--which simply can't be safe, even if we do want each line to have a max character count.

There are other problems -- there's no wrapping when we try to draw a line that won't fit in the screen space. It just overflows. We need to call string_length so we can wrap text lines not just by character count but by actual width.

I strongly suspect that some bizarre bug is causing the text buffer to try to render invalid characters, and this is breaking our sf::Font object somehow, leading to #454. Possibly storing the buffer lines as std::strings could fix this without us every having to understand an ungodly C-string encoding bug.