Instantiating a new font is expensive, both in CPU and
memory. Mitigate by adding a cache. On a hit, instead of instantiating
a new font, clone the one from the cache.
Remember, cloning is basically just a ref counter bump.
This allows us to a) move away from cairo's "toy" API, and b) let the
user specify font options in a single font "name" string:
Serif:size=10:weight=bold:slant=italic
This also allows us to simplify the font code significantly (except
for the fontconfig parts...); the font no longer sets itself in a
cairo surface - font users do that; the font simply returns a
cairo_scaled_font_t.
Furthermore, font_clone() has now been simplified to basically just
refcount the scaled font. I.e. there's no need to run the full
constructor and lookup and instantiate the cairo scaled font again.
