Also keep the text string around and drawing operations around and allow creating TTF_Text with no engine, for measurement or external rendering use.

Also added the ability to update text object properties (engine, font, text, etc.) dynamically at runtime.

This allows loading lots of text and making multiple font changes quickly. The application can call TTF_UpdateText() if there's a specific point where changes should be applied, otherwise they'll be applied as needed when the text is renderered.