Doxygen produces some false-positive warnings for Matrix and Vector
classes, but the generated documentation is fine. Worked around the
warnings by using @copybrief and @copydetails instead of @copydoc.
The requirements are cross-referenced to dedicated page, similarly to
TODO and bug lists.
Also updated / improved documentation for some texture / image formats.
All HTML code and Doxygen shortcuts such as @c, @b and @em are now
rewritten using Markdown syntax, which makes it more readable.
Also updated Doxyfile. Doxygen 1.8 at least is required to generate
the documentation now.