FIX: macosx, always put timers on main thread

The main thread is the one that is running when started, so we
should put all timers we control on that thread. Otherwise we
need to start and control runloops on the other threads which may
or may not be started. This is particularly important for draw_idle()
calls that may be sent from a separate thread where work was being
done, but the draw should happen on the main thread still.

Author: greglucas

src/_macosx.m

@@ -1869,7 +1869,7 @@ static void context_cleanup(const void* info)
     CFTimeInterval interval;
     PyObject* py_interval = NULL, * py_single = NULL, * py_on_timer = NULL;
     int single;
-    runloop = CFRunLoopGetCurrent();
+    runloop = CFRunLoopGetMain();
     if (!runloop) {
         PyErr_SetString(PyExc_RuntimeError, "Failed to obtain run loop");
         return NULL;