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:
- Surface receives wl_callback.done for the previous frame. Surface is thawed.
- 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.
- Surface gets frozen
- Frame clock processes the update scheduled at 2. The surface is frozen, so paint is prevented. PHASE_PAINT is considered handled.
- Compositor emits wl_callback.done again. Surface is thawed.
- 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: #3750 (closed)