[perf] Reset global caches after every trace

I'd disabled this to look at cairo-qt performance, then forgot about it.
Be clean, cleanup globals -- this should fix the huge performance loss
when running in series multiple backends that need separate font caches.

perf/cairo-perf-trace.c

@@ -596,6 +596,11 @@ cairo_perf_trace (cairo_perf_t *perf,
 
     if (target->cleanup)
 	target->cleanup (closure);
+
+    cairo_debug_reset_static_data ();
+#if HAVE_FCFINI
+    FcFini ();
+#endif
 }
 
 static void