Fix: use ViewKeyCache in volumetric fog instead of manually creating MeshPipelineViewLayoutKey

[kfc35]

Objective

• Related view bindgroup layout changes can cause crash #24084
• Fixes atmosphere example after merging Create mesh view bind group layout on demand to remove unused bindings #23982

Solution

• As @beicause mentioned, volumetric frog was calculating MeshPipelineViewLayoutKey manually when it could just rely on the ViewKeyCache instead, which has centralized logic for calculating it in check_views_need_specialization (STBN is added in impl From<MeshPipelineKey> for MeshPipelineViewLayoutKey)

Testing

• cargo run --example atmosphere works now, no strobing!
• cargo run --example volumetric_fog also works
• cargo run --example scrolling_fog works