Change text box to return remaining text as UTF-8

Currently if using an AFM-font,Prawn's text box will return overflow text in Win1252 encoding, even though the user must provide their text in UTF-8 format. To render this overflowed text, the user needs to pass `:skip_encoding`, which is a fairly awkward workflow.

In an ideal setting, we should treat any transcoding to Win1252 (which isn't even supported by Ruby, so we implement it ourselves) as an implementation detail, and never allow the transcoded text to cross the public API boundary. In other words, Prawn should be UTF-8 in, UTF-8 out to keep things simple for users.

See related discussion on #777, and on #779.
TODO: This may not actually be a correct way of converting back into UTF-8. Needs closer investigation.