Clean up context_pool upon static data reset

In commit f46ba56d5b the static context
stash was replaced by a dynamic freed pool, which needs to be cleared
upon resets.

Fixes:
cairo.c:181: warning: ‘context_pool’ defined but not used

Reported-by: Uli Schlachter <psychon@znc.in>

src/cairo.c

@@ -233,6 +233,8 @@ _cairo_reset_static_data (void)
 	}
     }
     CAIRO_MUTEX_UNLOCK (_cairo_error_mutex);
+
+    _freed_pool_reset (&context_pool);
 }
 
 /**