This commit introduces a VFS plugin that exposes the glyphs and
metadata of a TrueType font as a pseudo file system. The TTF font data
is obtained from the VFS. The resulting pseudo file system is a
directory that contains the files 'glyphs', 'baseline', 'max_width',
and 'max_height'.
The counter part of the plugin is the 'Vfs_font' class that implements
the 'Text_painter::Font' interface by accessing the pseudo file system
as provided by the TTF VFS plugin.
Fixes#2740
