Got finally pissed with the requirement of setting AbstractFont::_size
manually and took this as opportunity to rework the internal plugin API
in a better way. Bumped the interface version, as this is binary
incompatible. MagnumFont doesn't support lineHeight yet.
The fonts is simply a file with character and glyph parameters
along with glyph cache image. There is also MagnumFontConverter, which
allows to create the font from any currently opened font.
Added tests for both, they both need active GL context due to GlyphCache
instance. MagnumFont doesn't have test for creating glyph cache yet.