Bugfix: reset last_timestamp for each event

Long description:

Reset last_timestamp to XCB_CURRENT_TIME whenever a new event comes in.
This should be set to the value of the timestamp for the incoming event
for any event that contains a timestamp during event handling. For
events that do not contain a timestamp, this avoids using an old
timestamp when rendering which can lead to focus problems when a
timestamp is used multiple times for handling different events.

fixes #1383

To apply this patch, use:
curl http://cr.i3wm.org/patch/667/raw.patch | git am

@@ -1232,6 +1232,10 @@ void handle_event(int type, xcb_generic_event_t *event) {

         return;

     }

+    /* reset last_timestamp to avoid using an old value for events that do not

+     * have a timestamp */

+    last_timestamp = XCB_CURRENT_TIME;

+

     switch (type) {

         case XCB_KEY_PRESS:

         case XCB_KEY_RELEASE: