This is a rudimentary - but working - port.
Glyph uploads are still using the old machinery, a bunch of functions
still exist that probably aren't necessary anymore and each glyph emits
its own node.
This will need to be improved in further commits.
Pass the glyph position into the glyph caching functions,
not just the glyph index. This allows us to cache different
images for different subpixel positions.
Add a setter for per-renderer debug flags, and use
them where possible. Some places don't have easy access
to a renderer, so this is not complete.
Also, use g_message instead of g_print throughout.
