Change the binary format of the unicode glyph. Add a block index,
which allows to only store the glyph that are defined, and to store
standard glyph more efficiently.
With this it's possible to add the unicode glyph that are beyond
0xffff
It also allows to save ~10% to the size of mod-unifont.so
2 228 792 bytes -> 1 994 264 bytes on x86_64

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>