surface: Always set PHASE_PAINT as pending when updates are scheduled

surface: Always set PHASE_PAINT as pending when updates are scheduled

At times (most often when closing subsurfaces that are scheduling
relayouts) the PHASE_PAINT handling gets broken with the following
sequence:

1. Surface receives wl_callback.done for the previous frame.
   Surface is thawed.
2. A new update on the surface is scheduled. PHASE_PAINT is
   requested directly on the frame clock. priv->pending_phase is
   left unset in the surface.
3. Surface gets frozen
4. Frame clock processes the update scheduled at 2. The surface
   is frozen, so paint is prevented. PHASE_PAINT is considered
   handled.
5. Compositor emits wl_callback.done again. Surface is thawed.
6. At this point the machinery is off
   - The surface didn't paint but has pending update regions
   - priv->draw_needed is set in the toplevel and other portions
     of the widget tree
   - So queueing redraws is ineffective at eventually calling
     gdk_surface_schedule_update() again on the toplevel surface.
   - We don't paint anymore, so this broken state is not flushed
     until other subsurface changes manage to schedule the missing
     update.

To fix this, always set PHASE_PAINT in priv->pending_phase when
doing gdk_surface_schedule_update(). If the frame clock turns
around before the surface is thawed, it will still be waiting to
be processed the next iteration.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/3750