We use a small cache of size 16 for patterns created from solid colors,
e.g. cairo_set_source_rgb(). This helps with toolkits that draw many
widgets using the same colour scheme.
The cache uses a static index variable, which itself acts like a cache
of size 1, remembering the most recently used colour. So repeated
lookups for the same colour hit immediately. If that fails, the cache
is searched linearly, and if that fails too, a new pattern is created
and a random member of the cache is evicted.
