Corrected printf call and varargs overloading in Font::draw

The "String().printf()" call returns a bool, so before any printf-based font call would print "1" instead of the desired output. Additionally, the non-printf methods were const and the printf methods were - making the printf methods const means they collide with the non-printf methods, so I added 'f' suffixes (note that before, printf methods would be called on non-const Font objects even if only one argument was provided, a potential security issue).