Fix 2D flicker by always dequeueing retained phase items (#25163)

[morr]

Objective

Fixes #25163.

queue_material2d_meshes is the only system that takes items out of the
retained 2D phases, and it returns early when render_material_instances
is empty. That is the frame the last entity using that material type gets
despawned, so its items never get removed and stay in the phase for good.

They also keep the batch_range they had when they were last batched,
since batch_and_prepare_sorted_render_phase only rewrites the range of
items that still have a RenderMesh2dInstance. render_range advances the
index by batch_range.len() after every draw, so an item left over from a
batch of three hides the two live items behind it. With equal sort keys the
order changes from frame to frame, which is why it shows up as flicker
instead of something permanently missing.

The early return is older than the dequeue loop (#17567 added it back when
the system only queued, #23083 added the removal). The 3D
queue_material_meshes doesn't have one.

Solution

Move the check below the dequeue loop and make it a continue. There is
still nothing to queue when no entity uses the material, but removal now
always happens.

Testing

In my own app after applying the patch tree shadows stopped flickering