pango: Create one context per instance and remove class mutex

pango: Create one context per instance and remove class mutex

PangoCairo is thread-safe as long as the context and fontmap are not
shared between threads. Previously each subclass had its own context and
a class mutex for this reason, but apart from hurting performance this
was also not completely safe yet: the same fontmap might've been used by
different classes from different threads as the thread-default fontmap
(at time of class initialization) was used.