Added Renderer::flush() and Renderer::finish().
Worked around Doxygen issues in Renderer documentation.

The text was displayed as if it was code block.
Added some TODOs.
Text: moved glyph caching into separate classes.

The API is similar for both basic and distance field caching.
Text: moved font size to abstract base.
Text: simplified font layouters, removed remaining public API.

Now the font classes are prepared to be plugins.