draw_text_overflow: fix wrong offsets for multibyte text.

[avm]

Won't work for surrogate pairs or probably even combining characters, but it's better than strlen() and 'text + offset'.

[jonas]

Thanks for catching this.

In the process of integrating this I ended up rewriting it to use the existing UTF8 utilities since the original patch didn't seem fix all my test cases. However, I've kept you as the author.

If you have improvements to the test cases feel free to paste them below or make a new PR.