A top-level font now has a list of fallback fonts. When a glyph cannot
be found, we try each fallback font in turn, until we either find one
that has the glyph, or until we've exhausted the list.
To make this actually work in practise (read: to make performance
acceptable), the cache is re-worked and is now populated on demand.
It also supports non-ASCII characters, by using the 4-byte unicode
character as index instead.
Since having an array that can be indexed by a 4-byte value isn't
really viable, we now have a simple hash table instead of an array.
These properties aren't always included in the pattern, and when they
aren't, trying to get them will fail.
This isn't an error, just fallback to a default value.
Configure cairo's 'options' with default values, then update the
FontConfig pattern. Finally, update the cairo options after having
retrieved the final FontConfig option.
