Fix 2d specialization w/ change lists.

[tychedelia]

Some fixes for 2d post #22966.

This is complicated because, unlike 3d, 2d material systems are generic over their material and not type erased. Which means we have to be defensive against all the weird scenarios that can happen when different material systems race against each other.

I am not confident this is correct. The ultimate solution is just to get rid of all these bespoke 2d systems.

The main fixes are:

• material_bind_group_id is no longer reset to default every frame. extract_mesh2d was setting material_bind_group_id on every extraction. Previously this was overwritten later during queuing every frame, but with dirty tracking, entities aren't re-queued every frame so the default sticks.
• prepare_for_new_frame moved to a non-generic system.
• More careful about not adding wrong material type entities to the pending queue where they'll churn forever without getting specialized.
• Dequeue no longer cross-type stomps. When a view is dirty, we were returning all visible entities, which was problematic in the context of multiple materials.

[pcwalton]

Thanks, I hate it :)

We really ought to type-erase 2D materials, or just unify them with 3D.

[tychedelia]

We really ought to type-erase 2D materials, or just unify them with 3D.

That's the goal! Working on it now.

[atlv24]

oh my