wlt: toolkit: fix resize window stuck in need_resize=true

wlt: toolkit: fix resize window stuck in need_resize=true

The problem is that:
 wnd->w_frame contains a number
and
 wnd->need_frame=true
Therefore schedule_frame() doesn't schedule the callback.
But deleting these checks causes flicker. Further redesign
of the frame sheduling was necessary.

4. frame_callback()
3. do_frame()
3. idle_frame()
2. schedule_frame()
1. wlt_window_set_size()

Please, observe the need_resize variable.
-------------------------------------------------------------------------------
BAD (window stuck on resize):
$3 = {ref = 1, list = {next = 0x659c70, prev = 0x659c70},
  buffer_attached = true, skip_damage = false, need_resize = true,
  w_frame = 0x70d9b0, widget_list = {next = 0x686cb0, prev = 0x6b54d0}}

OK (window not stuck on resize):
$3 = {ref = 1, list = {next = 0x240cc70, prev = 0x240cc70},
  buffer_attached = true, skip_damage = false, need_resize = false,
  w_frame = 0x241bb10, widget_list = {next = 0x2439cb0, prev = 0x2468560}}

-----------------------------------------------------------------------------------

This is the basic idea:

-> time ->
frame callbacks()      *           *           *           *           *           *
wnd->idle_pending ------   ---------   ---------    --------   ---------------------
wnd->need_frame   ---      --- ---     ---          ---        ------ ---
schedule_frame()  *        *   *       *            *          *  *   **
idle_frame()        *        *   *       *            *          *  *   *

This design follows similiar design in weston/clients/window.c

Kmscon                     Weston
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
wlt_window_do_redraw() ~~~ shm_surface_swap()
do_frame() ~~~~~~~~~~~~~~~ window_attach_surface()

wnd->idle_pending ~~~~~~~~ window->redraw_scheduled

frame_callback()  ~~~~~~~~ frame_callback()

do_frame()  ~~~~~~~~~~~~~~ idle_redraw()

wlt_window_set_size()
wlt_window_schedule_redraw()
schedule_frame()  ~~~~~~~~ window_schedule_redraw()

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>