scratchpad: fix moving scratchpad window

From the source:

    When starting i3 initially (and after each change to the connected
    outputs), this function fixes the resolution of the __i3
    pseudo-output. When that resolution is not set to a function which
    shares a common divisor with every active output’s resolution,
    floating point calculation errors will lead to the scratchpad window
    moving when shown repeatedly.

fixes #632

src/handlers.c

@@ -212,6 +212,7 @@ static void handle_motion_notify(xcb_motion_notify_event_t *event) {
 
     Con *con;
     if ((con = con_by_frame_id(event->event)) == NULL) {
+        DLOG("MotionNotify for an unknown container, checking if it crosses screen boundaries.\n");
         check_crossing_screen_boundary(event->root_x, event->root_y);
         return;
     }
@@ -405,6 +406,8 @@ static void handle_screen_change(xcb_generic_event_t *e) {
 
     randr_query_outputs();
 
+    scratchpad_fix_resolution();
+
     ipc_send_event("output", I3_IPC_EVENT_OUTPUT, "{\"change\":\"unspecified\"}");
 
     return;