Bugfix: use the global root variable, don’t get the first one (Thanks quaec)

The case of an X11 server having multiple displays is handled correctly by the
code in src/mainx.c. However, due to some functions not being correctly
refactored and still getting the first screen (and also the first root window)
from the XCB connection, i3 was operating on the wrong root window.

src/floating.c

@@ -325,7 +325,6 @@ void floating_resize_window(Con *con, bool proportional,
 void drag_pointer(Con *con, xcb_button_press_event_t *event, xcb_window_t
                 confine_to, border_t border, callback_t callback, void *extra)
 {
-    xcb_window_t root = xcb_setup_roots_iterator(xcb_get_setup(conn)).data->root;
     uint32_t new_x, new_y;
     Rect old_rect;
     if (con != NULL)